Ensure field limitation is scoped to model. [ fix #2356 ]

CHANGELOG.md

@@ -7,6 +7,9 @@ For instructions on upgrading to newer versions, visit
 
 ### Resolved Issues
 
+* \#2356 When limiting fields returned in queries via `only` ensure that the
+  limitation is scoped to the model.
+
 * \#2353 Allow `update_attribute` to properly handle aliased fields.
 
 * \#2348 Conversion of strings to times should raise an arugment error if the

lib/mongoid/contextual/mongo.rb

@@ -599,11 +599,11 @@ def apply_options
       def selecting
         begin
           unless criteria.options[:fields].blank?
-            Threaded.selection = criteria.options[:fields]
+            Threaded.set_selection(klass, criteria.options[:fields])
           end
           yield
         ensure
-          Threaded.selection = nil
+          Threaded.set_selection(klass, nil)
         end
       end
 

lib/mongoid/fields/standard.rb

@@ -50,7 +50,7 @@ def constraint
       #
       # @since 2.1.8
       def eval_default(doc)
-        if fields = Threaded.selection
+        if fields = Threaded.selection(doc.class)
           evaluated_default(doc) if included?(fields)
         else
           evaluated_default(doc)

lib/mongoid/threaded.rb

@@ -301,25 +301,28 @@ def set_persistence_options(klass, options)
     # @example Get the field selection options.
     #   Threaded.selection
     #
+    # @param [ Class ] klass The model class.
+    #
     # @return [ Hash ] The field selection.
     #
     # @since 2.4.4
-    def selection
-      Thread.current["[mongoid]:selection"]
+    def selection(klass)
+      Thread.current["[mongoid][#{klass}]:selection"]
     end
 
     # Set the field selection on the current thread.
     #
     # @example Set the field selection.
-    #   Threaded.selection = { field: 1 }
+    #   Threaded.set_selection(Person, { field: 1 })
     #
+    # @param [ Class ] klass The model Class.
     # @param [ Hash ] value The current field selection.
     #
     # @return [ Hash ] The field selection.
     #
     # @since 2.4.4
-    def selection=(value)
-      Thread.current["[mongoid]:selection"] = value
+    def set_selection(klass, value)
+      Thread.current["[mongoid][#{klass}]:selection"] = value
     end
 
     # Get the global session override.

spec/mongoid/criteria_spec.rb

@@ -3248,6 +3248,19 @@ class C
           criteria.options[:fields]["_type"].should be_nil
         end
       end
+
+      context "when instantiating a class of another type inside the iteration" do
+
+        let(:criteria) do
+          Band.only(:name)
+        end
+
+        it "only limits the fields on the correct model" do
+          criteria.each do |band|
+            Person.new.age.should eq(100)
+          end
+        end
+      end
     end
 
     context "when using inheritance" do