Fix compatibility mistakes with ActiveRecord

lib/active_model/serializer.rb

@@ -198,8 +198,8 @@ module ActiveModel
         columns = klass.columns_hash
 
         attrs = _attributes.inject({}) do |hash, (name,key)|
-          column = columns[name]
-          hash.merge key => column[:type]
+          column = columns[name.to_s]
+          hash.merge key => column.type
         end
 
         associations = _associations.inject({}) do |hash, association|
@@ -212,7 +212,7 @@ module ActiveModel
 
       # The model class associated with this serializer.
       def model_class
-        name.sub(/Serializer$/, '')
+        name.sub(/Serializer$/, '').constantize
       end
 
       # Define how associations should be embedded.

test/serializer_test.rb

@@ -557,7 +557,7 @@ class SerializerTest < ActiveModel::TestCase
     Class.new do
       class << self
         def columns_hash
-          { :name => { :type => :string }, :age => { :type => :integer } }
+          { "name" => Struct.new(:type).new(:string), "age" => Struct.new(:type).new(:integer) }
         end
 
         def reflect_on_association(name)