MONGOID-5043 fix where query with custom primary key (#5170)

Author: neilshweky

lib/mongoid/fields/foreign_key.rb

@@ -60,6 +60,8 @@ def evolve(object)
         if object_id_field? || object.is_a?(Document)
           if association.polymorphic?
             association.convert_to_foreign_key(object)
+          elsif object.is_a?(Document) && object.respond_to?(association.primary_key)
+            primary_key_field.evolve(object.send(association.primary_key))
           else
             object.__evolve_object_id__
           end
@@ -142,6 +144,11 @@ def related_id_field
         @related_id_field ||= association.klass.fields["_id"]
       end
 
+      def primary_key_field
+        @primary_key_field ||= association.klass.fields[association.primary_key]
+      end
+
+
       # This is used when default values need to be serialized. Most of the
       # time just return the object.
       #

spec/integration/associations/foreign_key_spec.rb

@@ -27,6 +27,15 @@
       expect(email.company.id).to eq(company.id)
     end
 
+    it "has the correct criteria" do
+      company = ForeignKeySpec::Company.create!(c: "3")
+      email = ForeignKeySpec::Email.create!(company: company)
+
+      criteria = ForeignKeySpec::Email.where(company: company)
+
+      expect(criteria.selector).to eq({ "c_ref" => "3" })
+    end
+
     context 'with default scope' do
       context 'using default scope' do
         it 'looks up child' do