I'm considering implementing a feature by which a collection
association might limit the results.
By default, a collection association returns all values where the
foreign key on the `belongs_to` or `has_and_belongs_to` model matches
the parent object's primary key. These results can be filtered using the
`conditions` option, but that requires that other models have knowledge
of the parent model's table structure.
The feature I'm proposing is to add a `scope` option to collection
associations which takes a symbol representing a scope (or class mehtod)
defined on the associated model class.
A basic example would look like this:
class Comment
belongs_to :post
def self.visible
where(deleted_at: nil)
end
end
class Post
has_many :posts, scope: :visible
end
In the above example, `post.comments` would return `Comment` instances
whose `post_id = post.id` and whose `deleted_at = NULL`, providing basic
soft-deletion functionality.
While the same effect could be achieved with a declaration such as
`has_many :posts, conditions: ['deleted_at = ?', nil]`, that betrays
knowledge of the implementation of deletion on `Post` and would break if
the implementation were changed to a boolean value for deletion rather
than a timestamp field.
If I were to work on adding this functionality into ActiveRecord, is it
something that core might entertain merging?
Thank you,
Caleb Thompson
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