diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..0bbb62a
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,11 @@
+* text=auto
+
+/tests export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore
+/.php_cs export-ignore
+/.travis.yml export-ignore
+/phpunit.xml.dist export-ignore
+/CHANGELOG.md export-ignore
+/CONTRIBUTING.md export-ignore
+/README.md export-ignore
diff --git a/.travis.yml b/.travis.yml
index bb6e5c8..37af9cc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: php
 
 php:
+  - 5.5.9
   - 5.5
   - 5.6
   - 7.0
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 194e320..0110ce6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,23 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [0.4.1] - 2017-03-06
+### Fixed
+- [#114](https://github.com/tobscure/json-api/pull/114): Do not call `getAttributes()` twice. ([@f3ath](https://github.com/f3ath))
+
+## [0.4.0] - 2017-03-05
+### Added
+- [#96](https://github.com/tobscure/json-api/pull/96): Improve invalid parameter exceptions with more information about what's invalid. ([@nubs](https://github.com/nubs))
+
+### Changed
+- [#106](https://github.com/tobscure/json-api/pull/106): Convert snake_case into camelCase when calling a relationship method. ([@ustrugany](https://github.com/ustrugany))
+
+### Fixed
+- [#94](https://github.com/tobscure/json-api/pull/94): Properly support empty to-one relationships.
+- [#99](https://github.com/tobscure/json-api/pull/99): Omit attributes on resource identifier objects. ([@anthonyhendrickson](https://github.com/anthonyhendrickson))
+- [#103](https://github.com/tobscure/json-api/issues/103): Prevent resource relationships from being built twice per resource.
+- [#110](https://github.com/tobscure/json-api/pull/110): Omit empty attributes on resources. ([@f3ath](https://github.com/f3ath))
+
 ## [0.3.0] - 2016-03-31
 ### Added
 - [#79](https://github.com/tobscure/json-api/pull/79), [#81](https://github.com/tobscure/json-api/pull/81): Allow serializers to add links and metadata to resources. ([@bwaidelich](https://github.com/bwaidelich))
@@ -11,7 +28,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - [#85](https://github.com/tobscure/json-api/pull/85): Allow creation of relationships without data. ([@josephmcdermott](https://github.com/josephmcdermott))
 
 ### Fixed
-- [#65](https://github.com/tobscure/json-api/pull/65): Convert snake-case into camelCase when calling a relationship method. ([@avoelpel](https://github.com/avoelpel))
+- [#65](https://github.com/tobscure/json-api/pull/65): Convert kebab-case into camelCase when calling a relationship method. ([@avoelpel](https://github.com/avoelpel))
 - [#70](https://github.com/tobscure/json-api/pull/70): Include related resources even if relationship is not listed in sparse fieldset. ([@Damith88](https://github.com/Damith88))
 - [#72](https://github.com/tobscure/json-api/pull/72): Return `null` in `Parameters::getLimit` if no limit is set. ([@byCedric](https://github.com/byCedric))
 - [46142e5](https://github.com/tobscure/json-api/commit/46142e5823da3bebbd9dfc38833af4d808a5e3f3): Prevent primary "data" resources from showing up again in the "included" array. ([@tobscure](https://github.com/tobscure))
@@ -39,6 +56,8 @@ Completely rewrite to improve all the things.
 ## 0.1.0 - 2015-08-07
 - Initial release
 
+[0.4.1]: https://github.com/tobscure/json-api/compare/v0.4.0...v0.4.1
+[0.4.0]: https://github.com/tobscure/json-api/compare/v0.3.0...v0.4.0
 [0.3.0]: https://github.com/tobscure/json-api/compare/v0.2.1...v0.3.0
 [0.2.1]: https://github.com/tobscure/json-api/compare/v0.2.0...v0.2.1
 [0.2.0]: https://github.com/tobscure/json-api/compare/v0.1.1...v0.2.0
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 0ca2ed1..9a9f662 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -6,10 +6,9 @@ Contributions are welcome, and are accepted via pull requests. Please review the
 
 * Please follow the [PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md) and [PHP-FIG Naming Conventions](https://github.com/php-fig/fig-standards/blob/master/bylaws/002-psr-naming-conventions.md).
 * Ensure that the current tests pass, and if you've added something new, add the tests where relevant.
-* Remember that we follow [SemVer](http://semver.org). If you are changing the behaviour, or the public api, you may need to update the docs.
-* Send a coherent commit history, making sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash](http://git-scm.com/book/en/Git-Tools-Rewriting-History) them before submitting.
-* You may also need to [rebase](http://git-scm.com/book/en/Git-Branching-Rebasing) to avoid merge conflicts.
-
+* Remember that we follow [SemVer](http://semver.org). If you are changing the behavior, or the public api, you may need to update the docs.
+* Send a coherent commit history, making sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash](https://git-scm.com/book/en/Git-Tools-Rewriting-History) them before submitting.
+* You may also need to [rebase](https://git-scm.com/book/en/Git-Branching-Rebasing) to avoid merge conflicts.
 
 ## Running Tests
 
@@ -21,7 +20,7 @@ First, install the dependencies:
 $ composer install
 ```
 
-Then run phpunit:
+Then run PHPUnit:
 
 ```bash
 $ vendor/bin/phpunit
@@ -29,4 +28,4 @@ $ vendor/bin/phpunit
 
 If the test suite passes on your local machine you should be good to go.
 
-When you make a pull request, the tests will automatically be run again by [Travis CI](https://travis-ci.org/) on multiple php versions and hhvm.
+When you make a pull request, the tests will automatically be run again by [Travis CI](https://travis-ci.org/) on multiple PHP versions and HHVM.
diff --git a/README.md b/README.md
index 200ee23..3feee89 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,8 @@
 
 Works with version 1.0 of the spec.
 
+**⚠️ The project is no longer maintained.** Check out [tobyzerner/json-api-server](https://github.com/tobyzerner/json-api-server) instead.
+
 ## Install
 
 via Composer:
@@ -52,7 +54,7 @@ A JSON-API Document may contain one primary Element. The primary Element will be
 
 #### Sparse Fieldsets
 
-You can specify which fields (attributes and relationships) to be included on an Element using the `fields` method. You must provide a multidimensional array organized by resource type:
+You can specify which fields (attributes and relationships) are to be included on an Element using the `fields` method. You must provide a multidimensional array organized by resource type:
 
 ```php
 $collection->fields(['posts' => ['title', 'date']]);
@@ -102,6 +104,15 @@ public function comments($post)
 }
 ```
 
+By default, the `AbstractSerializer` will convert relationship names from `kebab-case` and `snake_case` into a `camelCase` method name and call that on the serializer. If you wish to customize this behaviour, you may override the `getRelationship` method:
+
+```php
+public function getRelationship($model, $name)
+{
+    // resolve Relationship called $name for $model
+}
+```
+
 ### Meta & Links
 
 The `Document`, `Resource`, and `Relationship` classes allow you to add meta information:
diff --git a/composer.json b/composer.json
index fbeebf8..3061728 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,7 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9"
+        "php": "^5.5.9 || ^7.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8 || ^5.0"
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 51bc73c..ae108af 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -8,6 +8,8 @@
          convertErrorsToExceptions="true"
          convertNoticesToExceptions="true"
          convertWarningsToExceptions="true"
+         failOnRisky="true"
+         failOnWarning="true"
          processIsolation="false"
          stopOnError="false"
          stopOnFailure="false"
diff --git a/src/AbstractSerializer.php b/src/AbstractSerializer.php
index 49a25c6..6e343cc 100644
--- a/src/AbstractSerializer.php
+++ b/src/AbstractSerializer.php
@@ -65,7 +65,7 @@ public function getMeta($model)
     /**
      * {@inheritdoc}
      *
-     * @throws LogicException
+     * @throws \LogicException
      */
     public function getRelationship($model, $name)
     {
@@ -85,9 +85,10 @@ public function getRelationship($model, $name)
     /**
      * Get the serializer method name for the given relationship.
      *
-     * kebab-case is converted into camelCase.
+     * snake_case and kebab-case are converted into camelCase.
      *
      * @param string $name
+     *
      * @return string
      */
     private function getRelationshipMethodName($name)
@@ -96,6 +97,10 @@ private function getRelationshipMethodName($name)
             $name = lcfirst(implode('', array_map('ucfirst', explode('-', $name))));
         }
 
+        if (stripos($name, '_')) {
+            $name = lcfirst(implode('', array_map('ucfirst', explode('_', $name))));
+        }
+
         return $name;
     }
 }
diff --git a/src/Collection.php b/src/Collection.php
index c0ae153..4d6707b 100644
--- a/src/Collection.php
+++ b/src/Collection.php
@@ -22,7 +22,7 @@ class Collection implements ElementInterface
      * Create a new collection instance.
      *
      * @param mixed $data
-     * @param SerializerInterface $serializer
+     * @param \Tobscure\JsonApi\SerializerInterface $serializer
      */
     public function __construct($data, SerializerInterface $serializer)
     {
@@ -34,7 +34,8 @@ public function __construct($data, SerializerInterface $serializer)
      *
      * @param mixed $data
      * @param SerializerInterface $serializer
-     * @return Resource[]
+     *
+     * @return \Tobscure\JsonApi\Resource[]
      */
     protected function buildResources($data, SerializerInterface $serializer)
     {
@@ -63,6 +64,8 @@ public function getResources()
      * Set the resources array.
      *
      * @param array $resources
+     *
+     * @return void
      */
     public function setResources($resources)
     {
@@ -73,6 +76,7 @@ public function setResources($resources)
      * Request a relationship to be included for all resources.
      *
      * @param string|array $relationships
+     *
      * @return $this
      */
     public function with($relationships)
@@ -84,25 +88,11 @@ public function with($relationships)
         return $this;
     }
 
-    /**
-     * Request a relationship to be identified for all resources.
-     *
-     * @param string|array $relationships
-     * @return $this
-     */
-    public function identify($relationships)
-    {
-        foreach ($this->resources as $resource) {
-            $resource->identify($relationships);
-        }
-
-        return $this;
-    }
-
     /**
      * Request a restricted set of fields.
      *
      * @param array|null $fields
+     *
      * @return $this
      */
     public function fields($fields)
diff --git a/src/Document.php b/src/Document.php
index 9d1318e..aa350ea 100644
--- a/src/Document.php
+++ b/src/Document.php
@@ -18,6 +18,8 @@ class Document implements JsonSerializable
     use LinksTrait;
     use MetaTrait;
 
+    const MEDIA_TYPE = 'application/vnd.api+json';
+
     /**
      * The included array.
      *
@@ -57,9 +59,10 @@ public function __construct(ElementInterface $data = null)
     /**
      * Get included resources.
      *
-     * @param ElementInterface $element
+     * @param \Tobscure\JsonApi\ElementInterface $element
      * @param bool $includeParent
-     * @return Resource[]
+     *
+     * @return \Tobscure\JsonApi\Resource[]
      */
     protected function getIncluded(ElementInterface $element, $includeParent = false)
     {
@@ -107,9 +110,10 @@ protected function getIncluded(ElementInterface $element, $includeParent = false
     }
 
     /**
-     * @param Resource[] $resources
-     * @param Resource $newResource
-     * @return Resource[]
+     * @param \Tobscure\JsonApi\Resource[] $resources
+     * @param \Tobscure\JsonApi\Resource $newResource
+     *
+     * @return \Tobscure\JsonApi\Resource[]
      */
     protected function mergeResource(array $resources, Resource $newResource)
     {
@@ -128,7 +132,8 @@ protected function mergeResource(array $resources, Resource $newResource)
     /**
      * Set the data object.
      *
-     * @param ElementInterface $element
+     * @param \Tobscure\JsonApi\ElementInterface $element
+     *
      * @return $this
      */
     public function setData(ElementInterface $element)
@@ -142,6 +147,7 @@ public function setData(ElementInterface $element)
      * Set the errors array.
      *
      * @param array $errors
+     *
      * @return $this
      */
     public function setErrors($errors)
@@ -155,6 +161,7 @@ public function setErrors($errors)
      * Set the jsonapi array.
      *
      * @param array $jsonapi
+     *
      * @return $this
      */
     public function setJsonapi($jsonapi)
diff --git a/src/ElementInterface.php b/src/ElementInterface.php
index 6b0ac00..81b74ba 100644
--- a/src/ElementInterface.php
+++ b/src/ElementInterface.php
@@ -38,6 +38,7 @@ public function toIdentifier();
      * Request a relationship to be included.
      *
      * @param string|array $relationships
+     *
      * @return $this
      */
     public function with($relationships);
@@ -46,6 +47,7 @@ public function with($relationships);
      * Request a restricted set of fields.
      *
      * @param array|null $fields
+     *
      * @return $this
      */
     public function fields($fields);
diff --git a/src/ErrorHandler.php b/src/ErrorHandler.php
index 33243f0..1e3492d 100644
--- a/src/ErrorHandler.php
+++ b/src/ErrorHandler.php
@@ -20,7 +20,7 @@ class ErrorHandler
     /**
      * Stores the valid handlers.
      *
-     * @var ExceptionHandlerInterface[]
+     * @var \Tobscure\JsonApi\Exception\Handler\ExceptionHandlerInterface[]
      */
     private $handlers = [];
 
@@ -28,8 +28,10 @@ class ErrorHandler
      * Handle the exception provided.
      *
      * @param Exception $e
-     * @return \Tobscure\JsonApi\Exception\Handler\ResponseBag
+     *
      * @throws RuntimeException
+     *
+     * @return \Tobscure\JsonApi\Exception\Handler\ResponseBag
      */
     public function handle(Exception $e)
     {
@@ -45,7 +47,9 @@ public function handle(Exception $e)
     /**
      * Register a new exception handler.
      *
-     * @param ExceptionHandlerInterface $handler
+     * @param \Tobscure\JsonApi\Exception\Handler\ExceptionHandlerInterface $handler
+     *
+     * @return void
      */
     public function registerHandler(ExceptionHandlerInterface $handler)
     {
diff --git a/src/Exception/Handler/ExceptionHandlerInterface.php b/src/Exception/Handler/ExceptionHandlerInterface.php
index 15ea0b4..4108fc0 100644
--- a/src/Exception/Handler/ExceptionHandlerInterface.php
+++ b/src/Exception/Handler/ExceptionHandlerInterface.php
@@ -19,7 +19,8 @@ interface ExceptionHandlerInterface
      * If the exception handler is able to format a response for the provided exception,
      * then the implementation should return true.
      *
-     * @param Exception $e
+     * @param \Exception $e
+     *
      * @return bool
      */
     public function manages(Exception $e);
@@ -27,8 +28,9 @@ public function manages(Exception $e);
     /**
      * Handle the provided exception.
      *
-     * @param Exception $e
-     * @return ResponseBag
+     * @param \Exception $e
+     *
+     * @return \Tobscure\JsonApi\Exception\Handler\ResponseBag
      */
     public function handle(Exception $e);
 }
diff --git a/src/Exception/Handler/FallbackExceptionHandler.php b/src/Exception/Handler/FallbackExceptionHandler.php
index 4d1fa96..2799d55 100644
--- a/src/Exception/Handler/FallbackExceptionHandler.php
+++ b/src/Exception/Handler/FallbackExceptionHandler.php
@@ -48,8 +48,9 @@ public function handle(Exception $e)
     }
 
     /**
-     * @param Exception $e
+     * @param \Exception $e
      * @param $status
+     *
      * @return array
      */
     private function constructError(Exception $e, $status)
diff --git a/src/Exception/Handler/InvalidParameterExceptionHandler.php b/src/Exception/Handler/InvalidParameterExceptionHandler.php
index d4ff608..374020a 100644
--- a/src/Exception/Handler/InvalidParameterExceptionHandler.php
+++ b/src/Exception/Handler/InvalidParameterExceptionHandler.php
@@ -32,6 +32,16 @@ public function handle(Exception $e)
         $status = 400;
         $error = [];
 
+        $code = $e->getCode();
+        if ($code) {
+            $error['code'] = $code;
+        }
+
+        $invalidParameter = $e->getInvalidParameter();
+        if ($invalidParameter) {
+            $error['source'] = ['parameter' => $invalidParameter];
+        }
+
         return new ResponseBag($status, [$error]);
     }
 }
diff --git a/src/Exception/InvalidParameterException.php b/src/Exception/InvalidParameterException.php
index b8fcf5e..72027c0 100644
--- a/src/Exception/InvalidParameterException.php
+++ b/src/Exception/InvalidParameterException.php
@@ -15,4 +15,28 @@
 
 class InvalidParameterException extends Exception
 {
+    /**
+     * @var string The parameter that caused this exception.
+     */
+    private $invalidParameter;
+
+    /**
+     * {@inheritdoc}
+     *
+     * @param string $invalidParameter The parameter that caused this exception.
+     */
+    public function __construct($message = '', $code = 0, $previous = null, $invalidParameter = '')
+    {
+        parent::__construct($message, $code, $previous);
+
+        $this->invalidParameter = $invalidParameter;
+    }
+
+    /**
+     * @return string The parameter that caused this exception.
+     */
+    public function getInvalidParameter()
+    {
+        return $this->invalidParameter;
+    }
 }
diff --git a/src/LinksTrait.php b/src/LinksTrait.php
index 316790b..1f22171 100644
--- a/src/LinksTrait.php
+++ b/src/LinksTrait.php
@@ -34,6 +34,7 @@ public function getLinks()
      * Set the links.
      *
      * @param array $links
+     *
      * @return $this
      */
     public function setLinks(array $links)
@@ -48,6 +49,7 @@ public function setLinks(array $links)
      *
      * @param string $key
      * @param string $value
+     *
      * @return $this
      */
     public function addLink($key, $value)
@@ -65,6 +67,8 @@ public function addLink($key, $value)
      * @param int $offset The current offset.
      * @param int $limit The current limit.
      * @param int|null $total The total number of results, or null if unknown.
+     *
+     * @return void
      */
     public function addPaginationLinks($url, array $queryParams, $offset, $limit, $total = null)
     {
@@ -95,6 +99,8 @@ public function addPaginationLinks($url, array $queryParams, $offset, $limit, $t
      * @param array $queryParams The query params provided in the request.
      * @param int $offset The offset to link to.
      * @param int $limit The current limit.
+     *
+     * @return void
      */
     protected function addPaginationLink($name, $url, array $queryParams, $offset, $limit)
     {
diff --git a/src/MetaTrait.php b/src/MetaTrait.php
index 5202033..5572446 100644
--- a/src/MetaTrait.php
+++ b/src/MetaTrait.php
@@ -34,6 +34,7 @@ public function getMeta()
      * Set the meta data array.
      *
      * @param array $meta
+     *
      * @return $this
      */
     public function setMeta(array $meta)
@@ -48,6 +49,7 @@ public function setMeta(array $meta)
      *
      * @param string $key
      * @param string $value
+     *
      * @return $this
      */
     public function addMeta($key, $value)
diff --git a/src/Parameters.php b/src/Parameters.php
index cfd6db6..0280e47 100644
--- a/src/Parameters.php
+++ b/src/Parameters.php
@@ -32,8 +32,10 @@ public function __construct(array $input)
      * Get the includes.
      *
      * @param array $available
+     *
+     * @throws \Tobscure\JsonApi\Exception\InvalidParameterException
+     *
      * @return array
-     * @throws InvalidParameterException
      */
     public function getInclude(array $available = [])
     {
@@ -43,7 +45,12 @@ public function getInclude(array $available = [])
             $invalid = array_diff($relationships, $available);
 
             if (count($invalid)) {
-                throw new InvalidParameterException('Invalid includes ['.implode(',', $invalid).']');
+                throw new InvalidParameterException(
+                    'Invalid includes ['.implode(',', $invalid).']',
+                    1,
+                    null,
+                    'include'
+                );
             }
 
             return $relationships;
@@ -56,8 +63,10 @@ public function getInclude(array $available = [])
      * Get number of offset.
      *
      * @param int|null $perPage
+     *
+     * @throws \Tobscure\JsonApi\Exception\InvalidParameterException
+     *
      * @return int
-     * @throws InvalidParameterException
      */
     public function getOffset($perPage = null)
     {
@@ -68,7 +77,7 @@ public function getOffset($perPage = null)
         $offset = (int) $this->getPage('offset');
 
         if ($offset < 0) {
-            throw new InvalidParameterException('page[offset] must be >=0');
+            throw new InvalidParameterException('page[offset] must be >=0', 2, null, 'page[offset]');
         }
 
         return $offset;
@@ -78,8 +87,10 @@ public function getOffset($perPage = null)
      * Calculate the offset based on the page[number] parameter.
      *
      * @param int $perPage
+     *
+     * @throws \Tobscure\JsonApi\Exception\InvalidParameterException
+     *
      * @return int
-     * @throws InvalidParameterException
      */
     protected function getOffsetFromNumber($perPage)
     {
@@ -96,6 +107,7 @@ protected function getOffsetFromNumber($perPage)
      * Get the limit.
      *
      * @param int|null $max
+     *
      * @return int|null
      */
     public function getLimit($max = null)
@@ -113,8 +125,10 @@ public function getLimit($max = null)
      * Get the sort.
      *
      * @param array $available
+     *
+     * @throws \Tobscure\JsonApi\Exception\InvalidParameterException
+     *
      * @return array
-     * @throws InvalidParameterException
      */
     public function getSort(array $available = [])
     {
@@ -137,7 +151,12 @@ public function getSort(array $available = [])
             $invalid = array_diff(array_keys($sort), $available);
 
             if (count($invalid)) {
-                throw new InvalidParameterException('Invalid sort fields ['.implode(',', $invalid).']');
+                throw new InvalidParameterException(
+                    'Invalid sort fields ['.implode(',', $invalid).']',
+                    3,
+                    null,
+                    'sort'
+                );
             }
         }
 
@@ -177,6 +196,7 @@ public function getFilter()
      *
      * @param string $key
      * @param null $default
+     *
      * @return mixed
      */
     protected function getInput($key, $default = null)
@@ -188,6 +208,7 @@ protected function getInput($key, $default = null)
      * Get the page.
      *
      * @param string $key
+     *
      * @return string
      */
     protected function getPage($key)
diff --git a/src/Relationship.php b/src/Relationship.php
index bbad37d..29f7fc6 100644
--- a/src/Relationship.php
+++ b/src/Relationship.php
@@ -19,14 +19,14 @@ class Relationship
     /**
      * The data object.
      *
-     * @var ElementInterface|null
+     * @var \Tobscure\JsonApi\ElementInterface|null
      */
     protected $data;
 
     /**
      * Create a new relationship.
      *
-     * @param ElementInterface|null $data
+     * @param \Tobscure\JsonApi\ElementInterface|null $data
      */
     public function __construct(ElementInterface $data = null)
     {
@@ -36,7 +36,7 @@ public function __construct(ElementInterface $data = null)
     /**
      * Get the data object.
      *
-     * @return ElementInterface|null
+     * @return \Tobscure\JsonApi\ElementInterface|null
      */
     public function getData()
     {
@@ -46,7 +46,8 @@ public function getData()
     /**
      * Set the data object.
      *
-     * @param ElementInterface|null $data
+     * @param \Tobscure\JsonApi\ElementInterface|null $data
+     *
      * @return $this
      */
     public function setData($data)
diff --git a/src/Resource.php b/src/Resource.php
index b8cabd9..b52230b 100644
--- a/src/Resource.php
+++ b/src/Resource.php
@@ -22,7 +22,7 @@ class Resource implements ElementInterface
     protected $data;
 
     /**
-     * @var SerializerInterface
+     * @var \Tobscure\JsonApi\SerializerInterface
      */
     protected $serializer;
 
@@ -43,13 +43,18 @@ class Resource implements ElementInterface
     /**
      * An array of Resources that should be merged into this one.
      *
-     * @var Resource[]
+     * @var \Tobscure\JsonApi\Resource[]
      */
     protected $merged = [];
 
+    /**
+     * @var \Tobscure\JsonApi\Relationship[]
+     */
+    private $relationships;
+
     /**
      * @param mixed $data
-     * @param SerializerInterface $serializer
+     * @param \Tobscure\JsonApi\SerializerInterface $serializer
      */
     public function __construct($data, SerializerInterface $serializer)
     {
@@ -72,7 +77,12 @@ public function toArray()
     {
         $array = $this->toIdentifier();
 
-        $array['attributes'] = $this->getAttributes();
+        if (! $this->isIdentifier()) {
+            $attributes = $this->getAttributes();
+            if ($attributes) {
+                $array['attributes'] = $attributes;
+            }
+        }
 
         $relationships = $this->getRelationshipsAsArray();
 
@@ -123,6 +133,10 @@ public function isIdentifier()
      */
     public function toIdentifier()
     {
+        if (! $this->data) {
+            return;
+        }
+
         $array = [
             'type' => $this->getType(),
             'id' => $this->getId()
@@ -194,6 +208,7 @@ protected function getOwnFields()
      * to the requested fieldset.
      *
      * @param array $fields
+     *
      * @return array
      */
     protected function filterFields(array $fields)
@@ -209,6 +224,7 @@ protected function filterFields(array $fields)
      * Merge the attributes of merged resources into an array of attributes.
      *
      * @param array $attributes
+     *
      * @return array
      */
     protected function mergeAttributes(array $attributes)
@@ -223,7 +239,7 @@ protected function mergeAttributes(array $attributes)
     /**
      * Get the resource relationships.
      *
-     * @return Relationship[]
+     * @return \Tobscure\JsonApi\Relationship[]
      */
     public function getRelationships()
     {
@@ -235,7 +251,7 @@ public function getRelationships()
     /**
      * Get the resource relationships without considering requested ones.
      *
-     * @return Relationship[]
+     * @return \Tobscure\JsonApi\Relationship[]
      */
     public function getUnfilteredRelationships()
     {
@@ -259,10 +275,14 @@ public function getRelationshipsAsArray()
     /**
      * Get an array of built relationships.
      *
-     * @return Relationship[]
+     * @return \Tobscure\JsonApi\Relationship[]
      */
     protected function buildRelationships()
     {
+        if (isset($this->relationships)) {
+            return $this->relationships;
+        }
+
         $paths = Util::parseRelationshipPaths($this->includes);
 
         $relationships = [];
@@ -280,7 +300,7 @@ protected function buildRelationships()
             }
         }
 
-        return $relationships;
+        return $this->relationships = $relationships;
     }
 
     /**
@@ -288,6 +308,7 @@ protected function buildRelationships()
      * relationships.
      *
      * @param array $relationships
+     *
      * @return array
      */
     protected function mergeRelationships(array $relationships)
@@ -302,7 +323,8 @@ protected function mergeRelationships(array $relationships)
     /**
      * Convert the given array of Relationship objects into an array.
      *
-     * @param Relationship[] $relationships
+     * @param \Tobscure\JsonApi\Relationship[] $relationships
+     *
      * @return array
      */
     protected function convertRelationshipsToArray(array $relationships)
@@ -315,7 +337,9 @@ protected function convertRelationshipsToArray(array $relationships)
     /**
      * Merge a resource into this one.
      *
-     * @param Resource $resource
+     * @param \Tobscure\JsonApi\Resource $resource
+     *
+     * @return void
      */
     public function merge(Resource $resource)
     {
@@ -329,6 +353,8 @@ public function with($relationships)
     {
         $this->includes = array_unique(array_merge($this->includes, (array) $relationships));
 
+        $this->relationships = null;
+
         return $this;
     }
 
@@ -352,6 +378,8 @@ public function getData()
 
     /**
      * @param mixed $data
+     *
+     * @return void
      */
     public function setData($data)
     {
@@ -359,7 +387,7 @@ public function setData($data)
     }
 
     /**
-     * @return SerializerInterface
+     * @return \Tobscure\JsonApi\SerializerInterface
      */
     public function getSerializer()
     {
@@ -367,7 +395,9 @@ public function getSerializer()
     }
 
     /**
-     * @param SerializerInterface $serializer
+     * @param \Tobscure\JsonApi\SerializerInterface $serializer
+     *
+     * @return void
      */
     public function setSerializer(SerializerInterface $serializer)
     {
diff --git a/src/SerializerInterface.php b/src/SerializerInterface.php
index ddd43cb..96730fa 100644
--- a/src/SerializerInterface.php
+++ b/src/SerializerInterface.php
@@ -17,6 +17,7 @@ interface SerializerInterface
      * Get the type.
      *
      * @param mixed $model
+     *
      * @return string
      */
     public function getType($model);
@@ -25,6 +26,7 @@ public function getType($model);
      * Get the id.
      *
      * @param mixed $model
+     *
      * @return string
      */
     public function getId($model);
@@ -34,6 +36,7 @@ public function getId($model);
      *
      * @param mixed $model
      * @param array|null $fields
+     *
      * @return array
      */
     public function getAttributes($model, array $fields = null);
@@ -42,6 +45,7 @@ public function getAttributes($model, array $fields = null);
      * Get the links array.
      *
      * @param mixed $model
+     *
      * @return array
      */
     public function getLinks($model);
@@ -50,6 +54,7 @@ public function getLinks($model);
      * Get the meta.
      *
      * @param mixed $model
+     *
      * @return array
      */
     public function getMeta($model);
@@ -59,6 +64,7 @@ public function getMeta($model);
      *
      * @param mixed $model
      * @param string $name
+     *
      * @return \Tobscure\JsonApi\Relationship|null
      */
     public function getRelationship($model, $name);
diff --git a/src/Util.php b/src/Util.php
index b5d6a28..cac77a3 100644
--- a/src/Util.php
+++ b/src/Util.php
@@ -26,6 +26,7 @@ class Util
      * ['user' => ['employer', 'employer.country'], 'comments' => []]
      *
      * @param array $paths
+     *
      * @return array
      */
     public static function parseRelationshipPaths(array $paths)
diff --git a/tests/AbstractSerializerTest.php b/tests/AbstractSerializerTest.php
index f0210ba..0ddc86e 100644
--- a/tests/AbstractSerializerTest.php
+++ b/tests/AbstractSerializerTest.php
@@ -14,6 +14,7 @@
 use Tobscure\JsonApi\AbstractSerializer;
 use Tobscure\JsonApi\Collection;
 use Tobscure\JsonApi\Relationship;
+use Tobscure\JsonApi\Resource;
 
 class AbstractSerializerTest extends AbstractTestCase
 {
@@ -41,6 +42,24 @@ public function testGetRelationshipReturnsRelationshipFromMethod()
         $this->assertTrue($relationship instanceof Relationship);
     }
 
+    public function testGetRelationshipReturnsRelationshipFromMethodUnderscored()
+    {
+        $serializer = new PostSerializer1;
+
+        $relationship = $serializer->getRelationship(null, 'parent_post');
+
+        $this->assertTrue($relationship instanceof Relationship);
+    }
+
+    public function testGetRelationshipReturnsRelationshipFromMethodKebabCase()
+    {
+        $serializer = new PostSerializer1;
+
+        $relationship = $serializer->getRelationship(null, 'parent-post');
+
+        $this->assertTrue($relationship instanceof Relationship);
+    }
+
     /**
      * @expectedException \LogicException
      */
@@ -68,6 +87,13 @@ public function comments($post)
         return new Relationship($element);
     }
 
+    public function parentPost($post)
+    {
+        $element = new Resource([], new self);
+
+        return new Relationship($element);
+    }
+
     public function invalid($post)
     {
         return 'invalid';
diff --git a/tests/DocumentTest.php b/tests/DocumentTest.php
index 6007c58..e1c6f51 100644
--- a/tests/DocumentTest.php
+++ b/tests/DocumentTest.php
@@ -12,7 +12,9 @@
 namespace Tobscure\Tests\JsonApi;
 
 use Tobscure\JsonApi\AbstractSerializer;
+use Tobscure\JsonApi\Collection;
 use Tobscure\JsonApi\Document;
+use Tobscure\JsonApi\Relationship;
 use Tobscure\JsonApi\Resource;
 
 /**
@@ -40,6 +42,37 @@ public function testItCanBeSerializedToJson()
     {
         $this->assertEquals('[]', (string) new Document());
     }
+
+    public function testToArrayIncludesIncludedResources()
+    {
+        $comment = (object) ['id' => 1, 'foo' => 'bar'];
+        $post = (object) ['id' => 1, 'foo' => 'bar', 'comments' => [$comment]];
+
+        $resource = new Resource($post, new PostSerializer2);
+        $includedResource = new Resource($comment, new CommentSerializer2);
+
+        $document = new Document($resource->with('comments'));
+
+        $this->assertEquals([
+            'data' => $resource->toArray(),
+            'included' => [
+                $includedResource->toArray()
+            ]
+        ], $document->toArray());
+    }
+
+    public function testNoEmptyAttributes()
+    {
+        $post = (object) [
+            'id' => 1,
+        ];
+
+        $resource = new Resource($post, new PostSerializerEmptyAttributes2);
+
+        $document = new Document($resource);
+
+        $this->assertEquals('{"data":{"type":"posts","id":"1"}}', (string) $document, 'Attributes should be omitted');
+    }
 }
 
 class PostSerializer2 extends AbstractSerializer
@@ -50,4 +83,27 @@ public function getAttributes($post, array $fields = null)
     {
         return ['foo' => $post->foo];
     }
+
+    public function comments($post)
+    {
+        return new Relationship(new Collection($post->comments, new CommentSerializer2));
+    }
+}
+
+class PostSerializerEmptyAttributes2 extends PostSerializer2
+{
+    public function getAttributes($post, array $fields = null)
+    {
+        return [];
+    }
+}
+
+class CommentSerializer2 extends AbstractSerializer
+{
+    protected $type = 'comments';
+
+    public function getAttributes($comment, array $fields = null)
+    {
+        return ['foo' => $comment->foo];
+    }
 }
diff --git a/tests/Exception/Handler/InvalidParameterExceptionHandlerTest.php b/tests/Exception/Handler/InvalidParameterExceptionHandlerTest.php
new file mode 100644
index 0000000..eb01ec2
--- /dev/null
+++ b/tests/Exception/Handler/InvalidParameterExceptionHandlerTest.php
@@ -0,0 +1,44 @@
+<?php
+
+/*
+ * This file is part of JSON-API.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Tobscure\Tests\Exception\Handler;
+
+use Exception;
+use Tobscure\JsonApi\Exception\Handler\InvalidParameterExceptionHandler;
+use Tobscure\JsonApi\Exception\Handler\ResponseBag;
+use Tobscure\JsonApi\Exception\InvalidParameterException;
+
+class InvalidParameterExceptionHandlerTest extends \PHPUnit_Framework_TestCase
+{
+    public function testHandlerCanManageInvalidParameterExceptions()
+    {
+        $handler = new InvalidParameterExceptionHandler();
+
+        $this->assertTrue($handler->manages(new InvalidParameterException));
+    }
+
+    public function testHandlerCanNotManageOtherExceptions()
+    {
+        $handler = new InvalidParameterExceptionHandler();
+
+        $this->assertFalse($handler->manages(new Exception));
+    }
+
+    public function testErrorHandling()
+    {
+        $handler = new InvalidParameterExceptionHandler();
+        $response = $handler->handle(new InvalidParameterException('error', 1, null, 'include'));
+
+        $this->assertInstanceOf(ResponseBag::class, $response);
+        $this->assertEquals(400, $response->getStatus());
+        $this->assertEquals([['code' => 1, 'source' => ['parameter' => 'include']]], $response->getErrors());
+    }
+}
diff --git a/tests/ParametersTest.php b/tests/ParametersTest.php
index a2f0bbe..e38362b 100644
--- a/tests/ParametersTest.php
+++ b/tests/ParametersTest.php
@@ -34,6 +34,17 @@ public function testGetIncludeReturnsEmptyArray()
         $this->assertEquals([], $parameters->getInclude(['posts', 'images']));
     }
 
+    /**
+     * @expectedException \Tobscure\JsonApi\Exception\InvalidParameterException
+     * @expectedExceptionCode 1
+     */
+    public function testGetIncludeWithUnallowedField()
+    {
+        $parameters = new Parameters(['include' => 'posts,images']);
+
+        $parameters->getInclude(['posts']);
+    }
+
     public function testGetSortReturnsArrayOfFieldToSortDirection()
     {
         $parameters = new Parameters(['sort' => 'firstname']);
@@ -55,6 +66,17 @@ public function testGetSortDefaultsToEmptyArray()
         $this->assertEmpty($parameters->getSort());
     }
 
+    /**
+     * @expectedException \Tobscure\JsonApi\Exception\InvalidParameterException
+     * @expectedExceptionCode 3
+     */
+    public function testGetSortWithUnallowedField()
+    {
+        $parameters = new Parameters(['sort' => 'firstname,lastname']);
+
+        $parameters->getSort(['firstname']);
+    }
+
     public function testGetOffsetParsesThePageOffset()
     {
         $parameters = new Parameters(['page' => ['offset' => 10]]);
@@ -64,12 +86,13 @@ public function testGetOffsetParsesThePageOffset()
 
     /**
      * @expectedException \Tobscure\JsonApi\Exception\InvalidParameterException
+     * @expectedExceptionCode 2
      */
     public function testGetOffsetIsAtLeastZero()
     {
         $parameters = new Parameters(['page' => ['offset' => -5]]);
 
-        $this->assertEquals(0, $parameters->getOffset());
+        $parameters->getOffset();
     }
 
     public function testGetOffsetParsesThePageNumber()
diff --git a/tests/ResourceTest.php b/tests/ResourceTest.php
index b17ef78..63246ca 100644
--- a/tests/ResourceTest.php
+++ b/tests/ResourceTest.php
@@ -170,6 +170,44 @@ public function testMetaMergeWithSerializerLinks()
             ]
         ], $resource1->toArray());
     }
+
+    public function testEmptyToOneRelationships()
+    {
+        $post1 = (object) ['id' => '123', 'foo' => 'bar'];
+
+        $resource1 = new Resource($post1, new PostSerializer4());
+        $resource1->with('author');
+
+        $this->assertEquals([
+            'type' => 'posts',
+            'id' => '123',
+            'attributes' => [
+                'foo' => 'bar'
+            ],
+            'relationships' => [
+                'author' => ['data' => null]
+            ]
+        ], $resource1->toArray());
+    }
+
+    public function testEmptyToManyRelationships()
+    {
+        $post1 = (object) ['id' => '123', 'foo' => 'bar'];
+
+        $resource1 = new Resource($post1, new PostSerializer4());
+        $resource1->with('likes');
+
+        $this->assertEquals([
+            'type' => 'posts',
+            'id' => '123',
+            'attributes' => [
+                'foo' => 'bar'
+            ],
+            'relationships' => [
+                'likes' => ['data' => []]
+            ]
+        ], $resource1->toArray());
+    }
 }
 
 class PostSerializer4 extends AbstractSerializer
@@ -194,6 +232,16 @@ public function comments($post)
     {
         return new Relationship(new Collection($post->comments, new CommentSerializer));
     }
+
+    public function author($post)
+    {
+        return new Relationship(new Resource(null, new CommentSerializer));
+    }
+
+    public function likes($post)
+    {
+        return new Relationship(new Collection([], new CommentSerializer));
+    }
 }
 class PostSerializer4WithLinksAndMeta extends PostSerializer4
 {