What is the difference between using ":" and "@" in fields_for

forms, ruby-on-rails

Solution

It seems like `fields_for` can't figure out what the `player` association is for the `@players_to_teams` instance variable. You bypass that by explicitly specifying the name of the association (i.e., `:player`). Are you defining the association in both directions? I.e.:

class PlayersToTeam < ActiveRecord::Base
  has_one :player
end

class Player < ActiveRecord::Base
  belongs_to :players_to_team
end

Additionally, looking at the documentation for `fields_for`, the first parameter is called `record_name`, so it seems like Rails is expecting the name of the association rather than the association itself. But it's hard to tell exactly what that method does without looking further into the code, and some of their examples do indeed pass the association directly into `fields_for`.

Okay, I was able to clone your sample project and reproduce the error. I think I understand what's happening.

After your call to `accepts_nested_attributes_for`, you now have an instance method on your model named `player_attributes=`. This is in addition to the `player=` method that's normally defined for a `has_one` association. The `player_attributes=` method accepts a hash of attributes, whereas the `player=` method only accepts an actual `Player` object.

Here's an example of the text input generated when you called `fields_for @players_to_teams.player`:

<input name="players_to_team[player][name]" ... />

and here's that same input when calling `fields_for :player`:

<input name="players_to_team[player_attributes][name]" ... />

When you call `update_attributes` in your controller, the first example will call `player=`, while the second example will call `player_attributes=`. In both cases, the argument passed to the method is a hash (because `params` is ultimately just a hash of hashes).

That's why you were getting an `AssociationTypeMismatch`: you can't pass a hash to `player=`, only a `Player` object.

It appears that the only safe way to use `fields_for` with `accepts_nested_attributes_for` is by passing the name of the association and not the association itself.

So to answer your original question, the difference is that one works and the other doesn't :-)

Problem

I am setting up embedded forms in my rails app. This does not work ``` <h1>PlayersToTeams#edit</h1> <%= form_for @players_to_teams do |field| %> <%= field.fields_for @players_to_teams.player do |f| %> <%= f.label :IsActive %> <%= f.text_field :IsActive %> <% end %> <%= field.label :BT %> <%= field.text_field :BT %> <br/> <%= field.submit "Save", class: 'btn btn-primary' %> <% end %> ``` Gives me a `ActiveRecord::AssociationTypeMismatch` error. Notice the `@players_to_teams.player` in the `forms_for` line. This does work: ``` <h1>PlayersToTeams#edit</h1> <%= form_for @players_to_teams do |field| %> <%= field.fields_for :player do |f| %> <%= f.label :IsActive %> <%= f.text_field :IsActive %> <% end %> <%= field.label :BT %> <%= field.text_field :BT %> <br/> <%= field.submit "Save", class: 'btn btn-primary' %> <% end %> ``` Notice the `:player` call in the `fields_for` line. Whats the difference between using a symbol and using an instance? I would think I would want to use an instance in this case, but I guess not? Edit models: ``` class Player < ActiveRecord::Base has_many :players_to_teams has_many :teams, through: :players_to_teams end class PlayersToTeam < ActiveRecord::Base belongs_to :player belongs_to :team accepts_nested_attributes_for :player end ``` controller: ``` class PlayersToTeamsController < ApplicationController def edit @players_to_teams=PlayersToTeam.find(params[:id]) end def update @players_to_teams=PlayersToTeam.find(params[:id]) respond_to do |format| if @players_to_teams.update_attributes(params[:players_to_team]) format.html { redirect_to @players_to_teams, notice: 'Player_to_Team was successfully updated.' } format.json { head :no_content } else format.html { render action: "edit" } format.json { render json: @players_to_teams.errors, status: :unprocessable_entity } end end end end ``` Example Project Github: https://github.com/workmaster2n/embedded-form-errors

Original source