使用`:has_many:through`记录关联处理复选框表单

class User< ApplicationRecord
  has_many :user_roles
  has_many :roles, through: :user_roles
end

class Role< ApplicationRecord
  has_many :user_roles
  has_many :users, through: :user_roles
end

class UserRole< ApplicationRecord
  belongs_to :user
  belongs_to :role
end

<%= form_with(model: user, local: true) do |form| %>
  <!-- 其他表单字段 -->

  <div class="field">
    <%= form.label :role_ids, 'Roles' %>
    <%= form.collection_check_boxes(:role_ids, Role.all, :id, :name) do |b| %>
      <%= b.label(class: 'checkbox') { b.check_box + b.text } %>
    <% end %>
  </div>

  <%= form.submit %>
<% end %>

def user_params
  params.require(:user).permit(:name, :email, role_ids: [])
end

class CreateUserRoles< ActiveRecord::Migration[6.1]
  def change
    create_table :user_roles do |t|
      t.belongs_to :user, null: false, foreign_key: true
      t.belongs_to :role, null: false, foreign_key: true

      t.timestamps
    end
  end
end