Mocking ActiveRecord to test concerns.

Part three of my series on Rails 4 (last: {% url 8d91b663 %}, and {% url 7340aa65 %}) is about mocking ActiveRecord to test concerns and other modules in a more independent way. I was doing this long before Rails 4 was out but now that Rails 4 is promoting skinny models and controllers it might be good for me to explain how I test "concerns".

When I first started working with Rails I had always thought that a lot of Rails people put too much into one spot, and not enough was extractable as independent modules that had certain dependencies (like ActiveRecord or a certain column and such.) I knew I wasn't original in my idea that code needs decoupling (read: Django), but it seemed to me that it wasn't such a big idea within the Rails community at the time.

In Rails 4 we saw people talking about it, split up and turn a lot of it into concerns, make your code more independent (though I don't remember if that was part of the full message over making skinny models.) Still though, we never saw a way to mock ActiveRecord (that I was able to find.) So I ended up having to extract my oldest mocks over to Rails 4 so I that I could test the new (in path) concerns.

When I designed the helpers to mock ActiveRecord I knew that I wouldn't want it on every it or specify. I actually wanted it done early, and to remain constant until the end of all the tests for that spec, that meant, (1): I needed to extend instead of include, (2): I needed to use after(:all) instead of after(:each). (3): I didn't want to have to use metadata to have it extended and (4): It should clean up after itself. So I ended up with:

module RSpec
  module Helpers
    module ActiveRecordMocks

      # --
      # @param [String,Symbol] name the name of the model
      # Mock a model and create a table.
      # --
      def mock_active_record_model(name, &block)
        create_temp_table(name, &block)

        klass = Class.new(ActiveRecord::Base)
        Object.const_set("#{name}_table".camelize, klass).class_eval do
          self.table_name = "__#{name}_table"
        end
      end

      # --
      # Create a temporary table.
      # --
      def create_temp_table(table, &block)
        ActiveRecord::Migration.suppress_messages do
          ActiveRecord::Migration.create_table "__#{table}_table",
              :temporary => true do |t|

            block.call(t)
          end
        end

        after :all do
          ActiveRecord::Migration.suppress_messages do
            ActiveRecord::Migration.drop_table "__#{table}_table"
          end
        end
      end
    end
  end
end

The idea behind was that if I wanted or needed a temporary table it would probably be for all the specs in that describe block. So I would need to do:

describe MyConcern do
  mock_active_record_model(:table_name) do |t|
    t.text(:my_row)
  end

  it 'should have an available ActiveRecord' do
    expect { TableNameTable.new }.to_not(raise_error)
  end
end

It also needed to try to attempt to prevent clashes inside of the database, so I also prefix everything with __ and suffix it with _table, this way we try not to clash with anything... but surely it wouldn't be hard to make it random since I always return both the Model and Table Name.

You can see it in this interaction:

[1] pry(#)> mock_active_record_model(:my_active_record_concern) do |t|
[1] pry(#)*   t.string(:hello)
[1] pry(#)* end

# => [
#  MyActiveRecordConcernTable(),
#  "__my_active_record_concern_table"
# ]

Note: Don't pay attention if it says the table does not exist because it's temporary.***

Since I didn't want to have to use metadata to have it extend the base class it's going into, I also needed to straight up config.extend instead of config.extend :condition => true (note the :condition => true it's the default in rspec 3)

RSpec.configure do |config|
  config.extend RSpec::Helpers::ActiveRecordMocks
end

The back story on why.

Creating the helpers to Mock

Tying it into RSpec