introduce a fake AR adapter for mocking database return values

activerecord/test/active_record/connection_adapters/fake_adapter.rb

+module ActiveRecord
+  class Base
+    def self.fake_connection(config)
+      ConnectionAdapters::FakeAdapter.new nil, logger
+    end
+  end
+
+  module ConnectionAdapters
+    class FakeAdapter < AbstractAdapter
+      attr_accessor :tables, :primary_keys
+
+      def initialize(connection, logger)
+        super
+        @tables       = []
+        @primary_keys = {}
+        @columns      = Hash.new { |h,k| h[k] = [] }
+      end
+
+      def primary_key(table)
+        @primary_keys[table]
+      end
+
+      def merge_column(table_name, name, sql_type = nil, options = {})
+        @columns[table_name] << ActiveRecord::ConnectionAdapters::Column.new(
+          name.to_s,
+          options[:default],
+          sql_type.to_s,
+          options[:null])
+      end
+
+      def columns(table_name, message)
+        @columns[table_name]
+      end
+    end
+  end
+end

activerecord/test/models/contact.rb

 class Contact < ActiveRecord::Base
-  def self.columns
-    @columns
-  end
+  establish_connection(:adapter => 'fake')
+
+  connection.tables = ['contacts']
+  connection.primary_keys = {
+    'contacts' => 'id'
+  }
 
   # mock out self.columns so no pesky db is needed for these tests
   def self.column(name, sql_type = nil, options = {})
-    @columns ||= []
-    @columns << ActiveRecord::ConnectionAdapters::Column.new(name.to_s, options[:default], sql_type.to_s, options[:null])
+    connection.merge_column('contacts', name, sql_type, options)
   end
 
   column :name,        :string