Updating Child Records

rubyonrails-talk
1

I have a 3 level hierarchical data structure, for this example lets say it's Bike (has many) Wheels (has many) Spokes.

Here's what I want to do, update a bunch of Spokes that all belong to one Bike but do it all with one Save! method call.

I've tried loading the Bike, accessing, changing, and adding Spokes through the Bike.wheels.spokes collections then doing Bike.save! but it doesn't work (the database remains unchanged). It DOES work however when I create a new bike record, add the wheels and spokes then save that.

So is there any way to have all the Spoke records saved with one call to save? I'm attempting to limit the number of individual SQL calls, cause there are a lot of spoke records and it will happen often.

Thanks in advance, Danny

2

The problem with bike.wheels.spokes is because bike has many wheels, and each wheel has many spokes. With bike.wheels.spokes, you’re trying to call spokes on the collection of all wheels for that bike, so it isn’t going to work! What you need to do is to find a single wheel, something a simple as @ bike.wheels.first will do, or do @wheel = Wheel.find(id) and just find the one wheel.

To add spokes:

@bike.wheels.first.spokes.build(:spoke_field_1 => “blah”)

To remove a spoke:

@bike.wheels.first.spokes

  • Spoke.find(spoke_id)

To edit a spoke:

@bike.wheels.first.spokes.find(id).update_attributes(params[:spoke])

3

Sorry, I should have been more clear, I'm aware of how the collections work, that was pseudo-code. I'm doing it properly in my real code.

#Pseudo code b = bike.find(id) then I loop through b.wheels    loop through wheel.spokes        spoke.thing = "newvalue"    end end

b.save!

4

I wouldn’t think that would work, because you’re not changing the associations themselves (i.e. adding/removing records), but you’re editing instances of the single records.

I can’t think of another way to do this, sorry. If it’s one value you could loop through and do update_attribute(“field”,“value”), and update_attributes which I guess you know about already.

5

I recommend (as I have in another thread) that you not do this.

Third normal form states that every column in a table should be dependent on the primary key. Well, in this case, one of the columns in the spokes table appears to be dependent not on its primary key, but on its associated bike's primary key.

Just my $.02.

///ark

6

It was a hypothetical example, things in my real DB are all correct and don't break any of the normal forms, one pk and one foreign key to link each record to it's parent.

According to Ryan's reply above, Save's are only propagated (to descendents) in Ruby model objects when there are insertions or deletions, not when the fields have been changed.

Thanks, Danny

7

It's true - they're done automatically on create, but not on update.

Ryan Bates covers this in his screencast on managing multiple models with 1 form (#75 Complex Forms Part 3 - RailsCasts). It's relatively simple to iterate through the child records using something like def save_wheels     wheels.each do |w|         w.save(false)     end   end

It assumes of course that you have updated their attributes and have a has_many :wheels in the Bike model.

and calling this method (of the Bike model) with a callback in the Bike model after_update :save_wheels

This way, the children are automatically saved when the parent is saved.

MC

8

eeew @ .save(false)

Take this as a warning to be very careful when using .save(false), passing in false skips all validations and you might not get desired results.