townx

Update: See the comments for an explanation of why I'm not doing validates_presence_of :project_id (instead of validates_presence_of :project). I'm prepared to be proved wrong on my assertions there, but they seem to hold up to my testing.

A common Rails pattern is to have two models with a has_many/belongs_to association, e.g.

class Task < ActiveRecord::Base
  belongs_to :project
  validates_presence_of :project
  validates_associated :project
end

class Project < ActiveRecord::Base
  has_many :tasks
  validates_presence_of :name
end

Then in the form for creating/editing a task, you might provide a drop-down box to select the associated project in the view, and display project error messages underneath it:

<p>Project: <%= collection_select :task, :project_id, Project.find(:all), :id, :name %></p>
<%= error_message_on :task, :project %>

Now if you try to save a task, you'll get the appropriate error message if you fail to select a project; also, if an invalid project gets assigned (e.g. invalid URL or incomplete Project instance), you'll get a validation error. I think this in uncontroversial.

BUT

If an error occurs on the project associated with the task, although you get an error message in the view, the drop-down box isn't highlighted (as it is with other fields). This is because the highlighting is done by looking at the errors object associated with the task, finding errors for any fields in the form with the same name, then altering the HTML for that field. So if there is an error for description and there's a field called task[description], that field gets an extra HTML <div> wrapped round it, e.g.

<div class="fieldWithErrors">
<input id="task_description" name="task[description]" size="30" 
type="text" value="" />
</div>

The issue with my task and project is that the validation errors are added for the projectattribute on the Task instance; but there isn't an HTML form field called task[project]: the drop-down is actually called task[project_id]. I tried to create a select element called task[project] instead, but this doesn't work (ActiveRecord complains that it is expecting a Project and you supplied a String).

My solution is to copy any project error for the Task to an error for project_id. I did this in the model, in the after_validation callback, like this:

class Task < ActiveRecord::Base
  belongs_to :project
  validates_presence_of :project
  validates_associate :project

  def after_validation
    project_error = errors.on(:project)
    errors.add(:project_id, project_error) if project_error
  end
end

This works fine: now any errors for the project are reflected onto project_id, so the task[project_id] drop-down box gets the error <div> (with class attribute set to "fieldWithErrors") wrapped around it. However, this doesn't feel quite right, and it makes me think I'm missing something. Is there some Rails voodoo I've overlooked which can be used to do this more cleanly?