[WIP] Extra conditions in JOIN statement

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Hi, I have a scenario where I want to add extra conditions in join statement apart from the foreign key column condition. I have gone through almost all of the relevant code of this gem but can't find any option to achieve that. Can anyone please guide me how to do that? I'm including an example

I have a model Item:

class Item
  has_many :custom_attribute_values, dependent: :destroy
end

And model CustomAttributeValue:

# id: int(11)
# value: varchar(255)
class CustomAttributeValue
end

I also have added a custom predicate in my config/initializers/ransack.rb

Ransack.configure do |config|
  config.add_predicate 'null_or_eq', arel_predicate: 'null_or_eq'
end

module Arel
  module Predications
    def null_or_eq(value)
      eq(value).or(eq(nil))
    end
  end
end

So I applied the following filter

filters = { 'custom_attribute_values_id_null_or_eq' => 4, 'custom_attribute_values_value_null' => true }
search_results = Item.all.ransack(filters)

calling search_results.result.to_sql returns the following SQL Query:

SELECT * FROM items
LEFT OUTER JOIN custom_attribute_values ON custom_attribute_values.item_id = items.id
WHERE ((custom_attribute_values.id IS NULL OR custom_attribute_values.id = 4) AND custom_attribute_values.value IS NULL)

However, I need the following SQL Query to be built to get the correct results:

SELECT * FROM items
LEFT OUTER JOIN custom_attribute_values ON custom_attribute_values.item_id = items.id
  AND custom_attribute_values.id = 4
WHERE (custom_attribute_values.value IS NULL)

Note that extra condition in JOIN statement custom_attribute_values.id = 4

Fixes #891

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