has_many through blowing away the association's metadata on mass association

Question

Hey, Not a Rails noob but this has stumped me. With has many through associations in Rails. When I mass assign wines to a winebar through a winelist association (or through) table with something like this.

class WineBarController

  def update

    @winebar = WineBar.find(params[:id])

    @winebar.wines = Wine.find(params[:wine_bar][:wine_ids].split(","))  // Mass assign wines.

    render (@winebar.update_attributes(params[:wine_bar]) ? :update_success : :update_failure) 

  end

end

This will delete every winelist row associated with that winebar. Then it finds all of the wines in wine_ids, which we presume is a comma separated string of wine ids. Then it inserts back into the winelist a new association. This would be expensive, but fine if the destroyed association rows didn't have metadata such as the individual wine bar's price per glass and bottle.

Is there a way to have it not blow everything away, just do an enumerable comparison of the arrays and insert delete whatever changes. I feel like that's something rails does and I'm just missing something obvious.

Thanks.

Answer 1

Your problem looks like it's with your first statement in the update method - you're creating a new wine bar record, instead of loading an existing record and updating it. That's why when you examine the record, there's nothing showing of the relationship. Rails is smart enough not to drop/create every record on the list, so don't worry about that.

If you're using the standard rails setup for your forms:

<% form_for @wine_bar do |f| %>

Then you can call your update like this:

class WineBarController
  def update
    @winebar = WineBar.find(params[:id])
    render (@winebar.update_attributes(params[:wine_bar]) ? :update_success : :update_failure) 
  end
end

You don't need to explicitly update your record with params[:wine_bar][:wine_ids], because when you updated it with params[:wine_bar], the wine_ids were included as part of that. I hope this helps!

UPDATE: You mentioned that this doesn't work because of how the forms are setup, but you can fix it easily. In your form, you'll want to rename the input field from wine_bar[wine_ids] to wine_bar[wine_ids_string]. Then you just need to create the accessors in your model, like so:

class WineBar < ActiveRecord::Base
  def wine_ids_string
    wines.map(&:id).join(',')
  end

  def wine_ids_string= id_string
    self.wine_ids = id_string.split(/,/)
  end
end

The first method above is the "getter" - it takes the list of associated wine ids and converts them to a string that the form can use. The next method is the "setter", and it accepts a comma-delimited string of ids, and breaks it up into the array that wine_ids= accepts.

You might also be interested in my article Dynamic Form Elements in Rails, which outlines how rails form inputs aren't limited to the attributes in the database record. Any pair of accessor methods can be used.