ActiveRecord associations with a conditions

ActiveRecord as a neat trick/ability which a lot of folks are not aware of.

You can pass a block to association, for example:

class Account < ApplicationRecord
 has_many :users, -> { where(admin: true) }
end

class User < ApplicationRecord
 belongs_to :account
end

What makes this even more interesting is that you can have multiple associations to the same model with different name and conditions.

class Account < ApplicationRecord
 has_many :users
 has_many :admin_users, -> { where(admin: true) }, class_name: "User"
end

Calling `@account.users` will return collection of all users, but calling `@account.admin_users` will return collection of only admin users.

To make things even more interesting, when creating a new record through the association will comply with the association condition.

@account.admin_users.create(name: "Admin User")

creates a new user with `admin` set to `true`.

Since, condition executes in the context of the association object, you can also use scopes.

class Account < ApplicationRecord
  has_many :users
  has_many :admin_users, -> { admins }, class_name: "User"
end

class User < ApplicationRecord
 belongs_to :account
 
 scope :admins, -> { where(admin: true) }
end

@account.admin_users # Works the same as examples above

This associations will also work for eager loading also, calling
`Account.includes(:admin_users)` will eager load admin users.

ActiveRecord can accomplish this because associations are built from `Relation` objects, and you can use `Relation` syntax to customize them.

Isn't this neat!

ps
This post is also published on our company dev blog