model_builder.rb

module ModelBuilder
  def self.included(spec)
    spec.before(:each) do
      @created_tables    = []
      @defined_constants = []
    end
    spec.after(:each) { undefine_constants }
    spec.after(:each) { drop_created_tables }
    spec.send(:include, Helpers)
  end

  module Helpers
    def create_table(table_name, &block)
      connection = ActiveRecord::Base.connection

      begin
        connection.execute("DROP TABLE IF EXISTS #{table_name}")
        connection.create_table(table_name, &block)
        @created_tables ||= []
        @created_tables << table_name
        connection
      rescue Exception => e
        connection.execute("DROP TABLE IF EXISTS #{table_name}")
        raise e
      end
    end

    def define_constant(class_name, base, &block)
      class_name = class_name.to_s.camelize

      klass = Class.new(base)
      Object.const_set(class_name, klass)

      klass.class_eval(&block) if block_given?

      @defined_constants ||= []
      @defined_constants << class_name

      klass
    end

    def define_model_class(class_name, &block)
      define_constant(class_name, ActiveRecord::Base, &block)
    end

    def define_model(name, columns = {}, &block)
      class_name = name.to_s.pluralize.classify
      table_name = class_name.tableize

      create_table(table_name) do |table|
        columns.each do |name, type|
          table.column name, type
        end
      end

      define_model_class(class_name, &block)
    end

    def undefine_constants
      @defined_constants.each do |class_name|
        Object.send(:remove_const, class_name)
      end
    end

    def drop_created_tables
      @created_tables.each do |table_name|
        ActiveRecord::Base.
          connection.
          execute("DROP TABLE IF EXISTS #{table_name}")
      end
    end
  end
end