Set the inverse when association queries are refined

Suppose Man has_many interests, and inverse_of is used.

Man.first.interests.first.man will correctly execute two queries,
avoiding the need for a third query when Interest#man is called. This is
because CollectionAssociation#first calls set_inverse_instance.

However Man.first.interests.where("1=1").first.man will execute three
queries, even though this is obviously a subset of the records in the
association.

This is because calling where("1=1") spawns a new Relation object from
the CollectionProxy object, and the Relation has no knowledge of the
association, so it cannot set the inverse instance.

This commit solves the problem by making relations spawned from
CollectionProxies return a new Relation subclass called
AssociationRelation, which does know about associations. Records loaded
from this class will get the inverse instance set properly.

Fixes #5717.

Live commit from La Conf! ✨

activerecord/lib/active_record.rb

@@ -76,6 +76,7 @@ module ActiveRecord
     autoload :AutosaveAssociation
 
     autoload :Relation
+    autoload :AssociationRelation
     autoload :NullRelation
 
     autoload_under 'relation' do

activerecord/lib/active_record/association_relation.rb

@@ -0,0 +1,14 @@
+module ActiveRecord
+  class AssociationRelation < Relation
+    def initialize(klass, table, association)
+      super(klass, table)
+      @association = association
+    end
+
+    private
+
+    def exec_queries
+      super.each { |r| @association.set_inverse_instance r }
+    end
+  end
+end

activerecord/lib/active_record/associations/association.rb

@@ -122,7 +122,11 @@ def klass
       # Can be overridden (i.e. in ThroughAssociation) to merge in other scopes (i.e. the
       # through association's scope)
       def target_scope
-        klass.all
+        all = klass.all
+        scope = AssociationRelation.new(klass, klass.arel_table, self)
+        scope.merge! all
+        scope.default_scoped = all.default_scoped?
+        scope
       end
 
       # Loads the \target if needed and returns it.

activerecord/test/cases/associations_test.rb

@@ -18,6 +18,8 @@
 require 'models/liquid'
 require 'models/molecule'
 require 'models/electron'
+require 'models/man'
+require 'models/interest'
 
 class AssociationsTest < ActiveRecord::TestCase
   fixtures :accounts, :companies, :developers, :projects, :developers_projects,
@@ -242,6 +244,17 @@ def test_scoped_allows_conditions
     david = developers(:david)
     assert david.projects.equal?(david.projects)
   end
+
+  test "inverses get set of subsets of the association" do
+    man = Man.create
+    man.interests.create
+
+    man = Man.find(man.id)
+
+    assert_queries(1) do
+      assert_equal man, man.interests.where("1=1").first.man
+    end
+  end
 end
 
 class OverridingAssociationsTest < ActiveRecord::TestCase