Rails：ActiveRecord :: AssociationTypeMismatch错误

Question

Is it possible to have the following two columns on an table and save them with active_record:

• user_name_id (which is an association)
• user_name (just text)

To demonstrate an attempt to save the association field user_name_id and the text field user_name

Here are the models:

#app/models/blog.rb
class Blog < ApplicationRecord
  belongs_to :user_name
end

#app/models/user_name.rb
class UserName < ApplicationRecord
  has_many :blogs
end

The form:

<%= form_for(blog) do |f| %>
  <% if blog.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(blog.errors.count, "error") %> prohibited this blog from being saved:</h2>

      <ul>
      <% blog.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :user_name %>
    <%= f.text_field :user_name %>
  </div>

  <div class="field">
    <%= f.label :user_name_id %>
    <%= f.collection_select :user_name_id, UserName.all, :id, :name %>
  </div>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

The strong params:

def blog_params
  params.require(:blog).permit(:user_name, :user_name_id)
end

When I submit the form in order to create a new blog , it errors out. The params hash looks ok:

“blog”=>{“user_name_id"=>"1", “user_name"=>”foo”}

However I am getting the following error:

ActiveRecord::AssociationTypeMismatch UserName(#70119900697480) expected, got String(#70119800086640)

Update : I understand that, ideally: the column name of one of these attributes should change. Nonetheless: is it possible to do this with rails? What would it take?

Answer 1

The issue is obvious from the answers above - change user_name attribute to be called something else.

As a very last resort, this might be a bad suggestion, but you could even try to declare which attribute you're using like:

class Blog < ApplicationRecord
  belongs_to :user_name, through: :user_name_id
end

But again, the reason you're associating the records, is so that you can call user_name at any point, through the association, and get all the information stored there. Meaning, you don't need to store the user_name with the blog... because you already are through the user_name association.

Answer 2

1. You should not use 'user_name' field at all if you already use belongs_to 'user_name'.
2. Please do not call Model 'UserName' this is not good, maybe this is UserProfile or just User?
3. Are you sure you want to cache user_name for your blog? If user will change his name it will makes many problems. If you want to cache user_name for blog anyway you can rename column on 'user_name_cached' and updates it on 'UserName' will updates on before_save callback.

UPDATE for authors' update:

belongs_to :your_new_belons_to_name, class_name: 'NameUser', foreign_key: 'name_user_id'

Answer 3

When you do:

belongs_to :user_name

ActiveRecord assumes that Blog has an attribute user_name_id (which it looks like you have - so far, so good). As stated in the Guide , you also get a setter method, user_name= which expects that you're going to pass it a UserName instance.

When you do something like:

Blog.new(“user_name_id"=>"1", “user_name"=>”foo”)

ActiveRecord is expecting user_name to be an instance of class UserName . But you just passed it a String . Which is exactly what the error is telling you.

Also, I agree with Alexey. I don't know why you would persist user_name (as a string) on Blog . If the class UserName has a name attribute, then you can always just do something like:

blog.user_name.name

I also agree with Alexey that UserName is a weird class name. Alexey's recommendations on naming are good ones, so please consider them.