Make schema not crash on computed attributes

We do not know the type for computed attributes, so we pick nil.

Perhaps at some point we might add a :type option for attributes (or
not), but in any case it's important to not crash when there are
computed attributes.

lib/active_model/serializer.rb

@@ -169,9 +169,15 @@ module ActiveModel
         klass = model_class
         columns = klass.columns_hash
 
-        attrs = _attributes.inject({}) do |hash, (name,key)|
-          column = columns[name.to_s]
-          hash.merge key => column.type
+        attrs = {}
+        _attributes.each do |name, key|
+          if column = columns[name.to_s]
+            attrs[key] = column.type
+          else
+            # Computed attribute (method on serializer or model). We cannot
+            # infer the type, so we put nil.
+            attrs[key] = nil
+          end
         end
 
         associations = _associations.inject({}) do |hash, (attr,association_class)|

test/serializer_test.rb

@@ -678,13 +678,16 @@ class SerializerTest < ActiveModel::TestCase
         define_method(:model_class) do model end
       end
 
-      attributes :name, :age
+      # Computed attribute; not a column.
+      def can_edit; end
+
+      attributes :name, :age, :can_edit
       has_many :posts, :serializer => Class.new
       has_one :parent, :serializer => Class.new
     end
 
     assert_equal serializer.schema, {
-      :attributes => { :name => :string, :age => :integer },
+      :attributes => { :name => :string, :age => :integer, :can_edit => nil },
       :associations => {
         :posts => { :has_many => :posts },
         :parent => { :belongs_to => :parent }