title: Rails 3: migration error when using json as a column type in an ActiveRecord backed by Postgres
body: <p>I am running Rails 3.2.17 and Postgres 9.3.4. I created a new ActiveRecord model using "rails generate" and one of the column types is json. My intention is to use the json column type in Postgres.</p>
  
  <p>The db migration contains this code:</p>
  
  <pre><code>class CreateThing &lt; ActiveRecord::Migration
    def change
      create_table :things do |t|
        t.integer :user_id
        t.json :json_data
        t.timestamps
      end
      add_index :things, :user_id
    end
  end
  </code></pre>
  
  <p>When I try to migrate with "rake db:migrate" I get this error:</p>
  
  <pre><code>-- create_table(:things)
  rake aborted!
  StandardError: An error has occurred, this and all later migrations canceled:
  
  undefined method `json' for #&lt;ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::TableDefinition:0x007fea3d465af8&gt;/Users/../db/migrate/20140425030855_create_things.rb:7:in `block in change'
  /Library/Ruby/Gems/2.0.0/gems/activerecord-3.2.17/lib/active_record/connection_adapters/abstract/schema_statements.rb:160:in `create_table'
  </code></pre>
  
  <p>Is this the right way to add a json column to an ActiveRecord? I cannot find any documentation or examples. Thanks!</p>