Java The Hutt

Many folks out there may not know this but you can set relations on AR objects in more than one way. Unless there's another I'm missing I know of 2 ways:

1. parent.child = Child.find(:first)




2. parent.child_id = Child.find(:first).id

When you save the parent object after each of the above statements you'll have created the parent-child relationship in the database. The second way is probably not as well known as the first, but it is very useful when updating a HABTM relationship.
Allow me to demonstrate...

In your view you have:

< %parent.children.each do |child|%>
< %=check_box_tag 'parent[child_ids][]', child.id, child_selected%> Child value < %=child.to_s%>
< %end%>

When you submit a form containing this field tag you'll receive all in your parameter set:

{'parent' => {'child_ids' => [2, 3, 7]}}

With which you can simply update the parent like so:

parent.attributes=(params[:parent])

And all of your related HABTM objects will be set and updated once you save the parent object.

With that groundwork laid have a look at the following code:

parent = Parent.new
parent.child = Child.find(1)
parent.child_id = 3
parent.save

What do you think will be saved in the database for this relation? The child with the id 1 or 3?

Of course the answer is 1. Huh? Don't get it? Yeah me neither. I stumbled across this bug, or inconsistency today while working on reassigning a related object to a new value.

I wrote this small unit test to reproduce what I was seeing in my app. The usage of setting "child_id" will work as long as the related object is null. I haven't checked yet to see if this is a bug or not and will update if it is already known. I'm using rails 1.1.6.

Be careful out there!