Fix disable_joins when using an enum STI type

When a model has an STI enum type, the type wasn't properly applied when
disable joins was set to true. In this case we need to apply the scope
from the association in `add_constraints` so that `type` is included in
the query. Otherwise in a `has_one :through` with `type` all records
will be returned because we're not filtering on type.

Long term I think this might be in the wrong place and that we want to
do this in a new definition of `target_scope` but that's a future
refactoring that needs to be done.

activerecord/lib/active_record/associations/disable_joins_association_scope.rb

@@ -32,6 +32,12 @@ module ActiveRecord
 
         def add_constraints(reflection, key, join_ids, owner, ordered)
           scope = reflection.build_scope(reflection.aliased_table).where(key => join_ids)
+
+          relation = reflection.klass.scope_for_association
+          scope.merge!(
+            relation.except(:select, :create_with, :includes, :preload, :eager_load, :joins, :left_outer_joins)
+          )
+
           scope = reflection.constraints.inject(scope) do |memo, scope_chain_item|
             item = eval_scope(reflection, scope_chain_item, owner)
             scope.unscope!(*item.unscope_values)

activerecord/test/cases/associations/has_one_through_disable_joins_associations_test.rb

@@ -8,9 +8,11 @@ require "models/project"
 require "models/developer"
 require "models/company"
 require "models/computer"
+require "models/club"
+require "models/membership"
 
 class HasOneThroughDisableJoinsAssociationsTest < ActiveRecord::TestCase
-  fixtures :members, :organizations, :projects, :developers, :companies
+  fixtures :members, :organizations, :projects, :developers, :companies, :clubs, :memberships
 
   def setup
     @member = members(:groucho)
@@ -61,4 +63,23 @@ class HasOneThroughDisableJoinsAssociationsTest < ActiveRecord::TestCase
       assert_no_match(/INNER JOIN/, nj)
     end
   end
+
+  def test_disable_joins_through_with_enum_type
+    joins = capture_sql { @member.club }
+    no_joins = capture_sql { @member.club_without_joins }
+
+    assert_equal 1, joins.size
+    assert_equal 2, no_joins.size
+
+    assert_match(/INNER JOIN/, joins.first)
+    no_joins.each do |nj|
+      assert_no_match(/INNER JOIN/, nj)
+    end
+
+    if current_adapter?(:Mysql2Adapter)
+      assert_match(/`memberships`.`type`/, no_joins.first)
+    else
+      assert_match(/"memberships"."type"/, no_joins.first)
+    end
+  end
 end

activerecord/test/models/member.rb

@@ -5,6 +5,7 @@ class Member < ActiveRecord::Base
   has_one :selected_membership
   has_one :membership
   has_one :club, through: :current_membership
+  has_one :club_without_joins, through: :current_membership, source: :club, disable_joins: true
   has_one :selected_club, through: :selected_membership, source: :club
   has_one :favorite_club, -> { where "memberships.favorite = ?", true }, through: :membership, source: :club
   has_one :hairy_club, -> { where clubs: { name: "Moustache and Eyebrow Fancier Club" } }, through: :membership, source: :club