Add option to specify custom method for has_many relationships creation

[chumakoff]

Currently has_many relations are added to a parent record via ActiveRecord's collection<< method. And there is no way to change that.

• collection<< doesn't always work for complex associations (especially for using through option)
• collection<< doesn't provide full control and confidence
• collection<< silently ignores invalid records
• customizing collection<< on model level (which is now the only workaround) for API purposes is not a good way

So, there should be a way to specify (and implement on API level) a custom method that appends has_many relationships.

Example

# post_resource.rb

has_many :tags, create_method: :add_tag

def add_tag(tag)
  @model.post_tags.create!(tag: tag) if @model.acceptable_tag?(tag)
end