Fix #3987.

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -102,6 +102,8 @@ module ActiveRecord
           @original_primary_key = @primary_key if defined?(@primary_key)
           @primary_key          = value && value.to_s
           @quoted_primary_key   = nil
+
+          connection.schema_cache.primary_keys[table_name] = @primary_key if connected?
         end
 
         def set_primary_key(value = nil, &block) #:nodoc:

activerecord/test/cases/primary_keys_test.rb

@@ -148,6 +148,16 @@ class PrimaryKeysTest < ActiveRecord::TestCase
     k.primary_key = "foo"
     assert_equal k.connection.quote_column_name("foo"), k.quoted_primary_key
   end
+
+  def test_set_primary_key_sets_schema_cache
+    klass = Class.new(ActiveRecord::Base)
+    klass.table_name = 'fuuuuuu'
+    klass.connection.create_table(:fuuuuuu, :id => false) { |t| t.integer :omg }
+    klass.primary_key = 'omg'
+    assert klass.connection.schema_cache.columns_hash['fuuuuuu']['omg'].primary
+  ensure
+    klass.connection.drop_table(:fuuuuuu) if klass.table_exists?
+  end
 end
 
 class PrimaryKeyWithNoConnectionTest < ActiveRecord::TestCase