为什么我不能使用参数创建 object？

Question

I've been suffering from a problem for several days. I can not create an object using the parameters from the form. My form:

<%= form_with(model: @battle, local: true) do |form| %>
<div class="field">
  <%= form.collection_select(:team_id, @teams, :id, :name) %>
</div>
  <%= form.submit 'Submit'%>
<% end %>

In this form i want to choose only 1 team.

controller:

  def create
    @battle = Battle.new
    @battle.user_id = current_user.id
    @battle.team_ids = params[:team_id]
    if @battle.save
      redirect_to root_path, notice: "Battle has been created"
    else
      render :new
    end
  end

  def battle_params
    params.require(:battle).permit(:team_id)
  end

And when used, this form creates an object without reference to the team.

logs:

Processing by BattlesController#create as HTML
  Parameters: {"authenticity_token"=>"0wWoFrQXYkEsXsMRgGyKi5Mde7WndhI6zYWY4KvhQlgdcCAaCZqH1z1z9dK0x91iqOPw/Jsb2T6Q+EVtGz4VsA==", "battle"=>{"team_id"=>"14"}, "commit"=>"Submit"}
  User Load (0.7ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2  [["id", 1], ["LIMIT", 1]]
  ↳ app/controllers/battles_controller.rb:14:in `create'
  Team Load (0.4ms)  SELECT "teams".* FROM "teams" WHERE 1=0
  ↳ app/controllers/battles_controller.rb:15:in `create'
   (0.4ms)  BEGIN
  ↳ app/controllers/battles_controller.rb:16:in `create'
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
  ↳ app/controllers/battles_controller.rb:16:in `create'
  Battle Create (3.8ms)  INSERT INTO "battles" ("user_id", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["user_id", 1], ["created_at", "2020-06-19 10:28:28.036147"], ["updated_at", "2020-06-19 10:28:28.036147"]]

If I try to create a battle with two teams, but it will be created with only one (without using parameters):

@battle.team_ids = [params[:team_id], 14]

Although, a battle is created without problems in the console:

battle = Battle.new
battle.team_ids = [13, 14]

I don't understand what the problem may be.

Answer 1

Instead of params[:team_id] try using battle_params[:team_id] instead:

@battle.team_ids = battle_params[:team_id]

otherwise you'd need to call params[:battle][:team_id] which is not the Rails way as it isn't as secure, but it would still work.