Don't build another association if one already exists - ruby-on-rails

Question

I have some problems getting a nested form working. In the below example I have a User that can define multiple custom labels for a post. The user should be able to enter a value for each particular label.
So one Post can have multiple labels, but should only have one value for each label! In example: Post can have a label named "Date" and also have a label named "Mood". For both labels there should be just one value.
The problem is when a User creates a Label -let say "Date"- it should only be possible to enter one value for this post for this particular label. So if a value for date is given, the form shouldn't build another field for date again. (the post already has a date)
User creates custom labels (this works)
On the edit page of the Post, User sees the labels he created in step 1 (this works)
User can enter a value for each of the Label (here is the problem)
I have the following models:
class User < ActiveRecord::Base
has_many :posts
has_many :labels
end
class Post < ActiveRecord::Base
has_many :label_values
belongs_to :user
accepts_nested_attributes_for :label_values, allow_destroy: true
end
class Label < ActiveRecord::Base
has_many :label_values
belongs_to :user
end
class LabelValue < ActiveRecord::Base
belongs_to :post
belongs_to :label
end
In my controller I have
class PostsController < ApplicationController
def edit
#labels = current_user.labels.all
#post.label_values.build
end
end
My form:
= simple_form_for #post do |f|
=f.fields_for :label_values do |s|
=s.association :label, :include_blank => false
=s.input :value
= f.button :submit
Like this, every time a User enters a value for a particular label, the next time a new label_value is build again and that is not what I want. For each label the User should be able to enter one value.

Answer 1

If you intend to use Label to attach metadata to Post you may want to consider using a relationship somewhat like the following:
class User < ActiveRecord::Base
has_many :user_labels # Labels which have been used by this user.
has_many :labels, through: :user_labels
has_many :posts
end
class Post < ActiveRecord::Base
has_many :post_labels
belongs_to :user, as: :author
has_many :labels, through: :post_labels
accepts_nested_attributes_for :post_labels
def available_labels
Label.where.not(id: labels.pluck(:label_id))
end
end
class Label < ActiveRecord::Base
# #attribute name [String] The name of the label
has_many :user_labels
has_many :post_labels
has_many :users, through: :user_labels
has_many :posts, through: :post_labels
validates_uniqueness_of :name
end
# Used to store label values that are attached to a post
class PostLabel < ActiveRecord::Base
belongs_to :post
belongs_to :label
validates_uniqueness_of :label, scope: :post # only one label per post
# #attribute value [String] the value of the label attached to a post
end
# Used for something like displaying the most used labels by a user
class UserLabel < ActiveRecord::Base
belongs_to :user
belongs_to :label
end
In this setup labels act somewhat like "tags" on Stackoverflow. Each separate label is unique in the system but may be attached to many posts - by many users.
Note the validates_uniqueness_of :label, scope: :post validation which ensures this.
The actuals values attached to a tag are stored on the value attribute of PostLabel. One major drawback to this approach is that you are really limited on by type of the PostLabel value column and may have to do typecasting. I really would not use it for dates as in your example as it will be difficult to do a query based on date.
def PostController
def new
#available_labels = Label.all()
end
def edit
#post = Post.joins(:labels).find(params[:id])
#available_labels = #post.available_labels
end
def create
#post = Post.new(create_params)
if (#post.save)
#post.labels.each do |l|
#post.user.labels << l
end
# ...
else
# ...
end
end
private
def create_params
params.permit(:post).allow(:post_labels)
end
end
Added
This is simply some opinionated recommendations:
To add new labels to a post I would add a text input below the already assigned labels.
I would autocomplete using GET /labels. and filter against the inputs already present in the form. You can allow users to create new labels on the fly by doing an ajax request to POST /labels.

Answer 2

I finally came to something. In the Post model I created:
def empty_labels
Label.where.not(:id => labelValue.select(:label_id).uniq)
end
def used_simfields
LabelValue.where(post_id: id)
end
Then in the view I did:
= simple_form_for #post do |f|
-#post.used_labels.each do |used_label|
=f.fields_for :label_values, used_label do |old_label|
=old_label :value
-#post.empty_labels.each do |empty_label|
=empty_label.fields_for :label_values, #post.label_values.new do |new_label|
=new_label.association :label
=new_label.input :value
I am very sure there are nicer ways to achieve this, so new ideas are very welcome.