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! :sparkles:

4 files changed, 33 insertions, 1 deletions

diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index 0330c0f37f..994cacb4f9 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -76,6 +76,7 @@ module ActiveRecord
     autoload :AutosaveAssociation
 
     autoload :Relation
+    autoload :AssociationRelation
     autoload :NullRelation
 
     autoload_under 'relation' do
diff --git a/activerecord/lib/active_record/association_relation.rb b/activerecord/lib/active_record/association_relation.rb
new file mode 100644
index 0000000000..e2976577f6
--- /dev/null
+++ b/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
diff --git a/activerecord/lib/active_record/associations/association.rb b/activerecord/lib/active_record/associations/association.rb
index 710babe024..608a6af16c 100644
--- a/activerecord/lib/active_record/associations/association.rb
+++ b/activerecord/lib/active_record/associations/association.rb
@@ -122,7 +122,11 @@ module ActiveRecord
       # 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.
diff --git a/activerecord/test/cases/associations_test.rb b/activerecord/test/cases/associations_test.rb
index 95c571fd03..0f2d22a4a2 100644
--- a/activerecord/test/cases/associations_test.rb
+++ b/activerecord/test/cases/associations_test.rb
@@ -18,6 +18,8 @@ require 'models/ship'
 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 @@ class AssociationProxyTest < ActiveRecord::TestCase
     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