Make it easier to set default associations in a Factory

[paulcsmith]

Depends on #331

class ArticleBox < Box
  def initialize
    post { PostBox.create }
  end

  # This is auto-generated by the box. Putting it here just to show rough implementation
  def post
    before_save do
      if post_id.value.blank?
        record_or_id = yield
        id = if record_or_id.is_a?(Avram::Model)
          record_or_id.id
        else
           record_or_id
        end
        post_id id
      end
    end
  end
end

This will most likely require changes to https://github.com/luckyframework/avram/blob/master/src/avram/save_operation.cr so that we have access to an ASSOCIATIONS constant that has a list of associations and the column to use.

Relevant examples:

• avram/src/avram/save_operation_template.cr

  Line 43 in bb57679

  add_column_attributes({{ primary_key_type }}, {{ columns }})

• avram/src/avram/save_operation.cr

  Lines 80 to 82 in d67dd63

  	{% for attribute in attributes %}
  	{% COLUMN_ATTRIBUTES << attribute %}
  	{% end %}

We'd want to add a add_association_attributes or something. This will be a bit more complex so I'll try to tackle it, but if someone else wants to take a stab at it, post here and I'll try to help if you run into anything

[matthewmcgarvey]

I'm looking into this

[paulcsmith]

Thanks for looking into this @matthewmcgarvey!

[jwoertink]

This is basically how it works now

class PostFactory < Avram::Factory
  def initialize
    before_save do
      if operation.user_id.value.nil?
        user(UserFactory.create)
      end
    end
  end
  def user(u : User)
    user_id(u.id)
  end
end

PostFactory.create

I'd imagine this could look like

class PostFactory < Avram::Factory
  belongs_to user : UserFactory
end

The tricky part might be if this is just a macro that ends up making an initialize, how do you handle when an initialize is defined by the user? 🤔