Redefine foreign key name for all associations in Rails - ruby-on-rails

Question

I have an STI model with many associations:
class MyModel < ActiveRecord::base
has_many :things
has_many :other_things
# ... a lot of `has_many`
end
Then I add non STI model as nested just to add some specific behaviour to MyModel without extending it directly:
class Nested < MyModel
self.inheritance_column = nil
end
But then my associations don't work. They have my_model_id column because they refer to MyModel and they should refer to Nested as well. But all these has_many's expect to use nested_id column as a foreign key (it depends on class name).
I could've typed inside class Nested:
has_many :things, foreign_key: 'my_model_id'
has_many :other_things, foreign_key: 'my_model_id'
But if it's possible, how to specify the foreign key for all the associations at once in Nested class?

Answer 1

If all your has_many associations are declared on MyModel, you should be fine, or you may benefit from upgrading Rails; the following works perfectly for me in 4.2.4. Rails uses the class that declares has_many to generate the foreign_key, so even when Nested inherits, :things still get looked up by my_model_id.
class MyModel < ActiveRecord::Base
has_many :things
end
class MyA < MyModel
end
class Nested < MyModel
self.inheritance_column = nil
end
class Thing < ActiveRecord::Base
belongs_to :my_model
end
> m = MyModel.create
=> #<MyModel id: 1, type: nil, created_at: "2015-12-22 02:21:16", updated_at: "2015-12-22 02:21:16">
> m.things.create
=> #<ActiveRecord::Associations::CollectionProxy [#<Thing id: 1, my_model_id: 1, created_at: "2015-12-22 02:21:19", updated_at: "2015-12-22 02:21:19">]>
> n = Nested.create
=> #<Nested id: 2, type: nil, created_at: "2015-12-22 02:21:27", updated_at: "2015-12-22 02:21:27">
> n.things.create
=> #<ActiveRecord::Associations::CollectionProxy [#<Thing id: 2, my_model_id: 2, created_at: "2015-12-22 02:21:32", updated_at: "2015-12-22 02:21:32">]>
> n.reload.things
Nested Load (0.2ms) SELECT "my_models".* FROM "my_models" WHERE "my_models"."id" = ? LIMIT 1 [["id", 2]]
Thing Load (0.1ms) SELECT "things".* FROM "things" WHERE "things"."my_model_id" = ? [["my_model_id", 2]]
=> #<ActiveRecord::Associations::CollectionProxy [#<Thing id: 2, my_model_id: 2, created_at: "2015-12-22 02:21:32", updated_at: "2015-12-22 02:21:32">]>
If you're giving Nested its own has_many associations, the code that generates foreign keys is pretty deep and inaccessible. You end up in the HasManyReflection, which either uses the foreign_key (or as) option declared on your has_many or derives it from the class name. There's no obvious way to customize those methods unless you do something inadvisable like override Nested.name.
foreign_key:
def foreign_key
#foreign_key ||= options[:foreign_key] || derive_foreign_key
end
derive_foreign_key:
def derive_foreign_key
if belongs_to?
"#{name}_id"
elsif options[:as]
"#{options[:as]}_id"
else
active_record.name.foreign_key
end
end
So the simplest way to go about this might just be a loop:
class Nested < MyModel
self.inheritance_column = nil
%i[things other_things].each do |association|
has_many association, foreign_key: 'my_model_id'
end
end
Or if you're feeling metaprogrammy, redefine has_many:
class Nested < MyModel
self.inheritance_column = nil
def self.has_many(klass, options={})
options.reverse_merge!(foreign_key: 'my_model_id')
super
end
has_many :things
has_many :other_things
end

Answer 2

My solution might be is not recommended, but it works. Here it is:
class Nested < MyModel
def self.name
MyModel.name
end
end
ActiveRecord will be looking for my_model_id foreign key for all the associations defined or redefined in Nested and MyModel classes.