AssociationScope

I always wondered, why there was no functionality to use associations not only on ActiveRecord objects but on scopes as well. When I have

current_user.topics # => #<ActiveRecord::Relation [...]>

why can't I use the same construction on a collection of users like

current_user.friends.topics # => #<ActiveRecord::Relation [...]>

and retrieve the collection of topics, my friends posted? Instead I wrote weird scopes like

class Topic < ApplicationRecord
  belongs_to :user
  scope :of_users, -> (users) { joins(:user).where(users: users) }
end

over and over again across all of my models to write something like Topic.of_users(current_user.friends) when I wanted to write current_user.friends.topics instead. And belongs_to is the easiest part.

When you have this problem, the AssociationScope gem is for you!

Installation

Add this line to your application's Gemfile:

gem 'association_scope'

And then execute:

$ bundle

Or install it yourself as:

$ gem install association_scope

Usage

After installation you can use has_association_scope_on in your models:

class Topic < ApplicationRecord
  belongs_to :user
  has_association_scope_on [:user]
end

Now you can use your associations as scopes and chain other scopes with them. You can write

Topic.all.users

to retrieve the users of all of the topics of your application.

Migration from .of_model

When you already use any form of .of_model scope, you can replace it with association scopes:

# replace
Topic.of_users(current_user.friends)
# with
current_user.friends.topics

When you chain scopes, you have to merge with the previous scope:

# replace
scope.of_users(users)
# with
users.topics.merge(scope)

Known Issues

• This gem works with reflections. To make this work, the has_association_scope_on call has to be below your association definitions.

# won't work
class Topic
  has_association_scope_on [:user]
  belongs_to :user
end

# works
class Topic
  belongs_to :user
  has_association_scope_on [:user]
end

• Does not work for tables without primary key.
• To use distinct on rows, all values of this row must be of types other than JSON. Workaround: Migrate JSON columns to JSONB.
• Error messages are not raised during application start, but on first instantiation, because of the order in which classes are loaded.

Development

Clone this repository and run bundle. To use rails console you have to navigate to the dummy application

$ cd spec/dummy