Rails 3 Associations Error

Rails Association error

You need to enable nested attributes for :occasions in your model:

class User ActiveRecord::Base
  has_many :occasions   
  accepts_nested_attributes_for :occasions
end

Otherwise the form builder does not not how to use the Occasion object that you're building in the controller, so it'll just use the :occasions symbol as "object", giving you this error.

rails error Class must exist - Associations

Rails 5 makes belongs_to association required by default. So you cannot create Post without associating it with a Travel

@post = Post.new(post_params)
@post.travel = travel
@post.save

If you want to make the association optional, you will have to mention it explicitly

class Post < ActiveRecord::Base
   belongs_to :travel, optional: true
end

Rails 4: has many through association error

Rails convention expects your through table to be named: collections_releases. And your model CollectionsRelease. Of course you could override if you wanted to keep the singular collection_releases name.

Rails 3 'association :[...] not found' error for has_many :through relationship

If you are working over PressRelease you should use <%= f.association :categories %>, change the :category for :categories, because of the relation.

Show association errors in a form

When you make the call to redirect_to product_path(@product), you are invoking a different controller action, namely the Products#show action. The instance variables your views will have been reset. You probably aren't initializing a @review instance variable in this action which is why you are getting an exception.

What you want is something like this

# posts_controller.rb
def show
  @product = Product.find(params[:product_id])
  @review = @product.reviews.build
end

# show.html.erb
= form_for([@product, @review]) do |f|
  -if @review.errors.any?
    .errors
      %h2
        =pluralize(@review.errors.count, "error")
      %ul
        =@review.errors.full_messages.each do |msg|
          %li
            =msg

# reviews_controller.rb
def create
  @product = Product.find(params[:product_id])
  @review = @product.reviews.build(params[:review])
  if @review.save
    redirect_to product_path(@product)
  else
    render 'products/show'
  end
end

The key is that if the review is unsuccessfully created, you will re-render the page with the same instance variables intact.

User has_many association not working (error:Could not find the association :user_categories in model Category)

I can't say for sure what the problem could be, but a few things I could point out:

1. UserCategory's accepts_nested_attributes_for, does that mean the you want to be able to dynamically create categories?

2. Category has_many :users, through: :user_categories, not user

3. You need to follow the Rails file naming conventions, user.rb, user_category.rb and category.rb

These may not be the problem/solution, but I believe they're in the way of resolving the problem.

Rails - belongs_to has_many association error

1. Is a belongs_to / has_many association correct for this situation? Should this be a has_one?

To setup a relationship where a user can only belong to a single group but groups have many users then you are on the right track.

class Group < ActiveRecord::Base
  has_many :users
end

class User < ActiveRecord::Base
  belongs_to :group
end

belongs_to places the foreign key column as users.group_id. Using has_one would place it on groups.user_id column which would only allow a one to one mapping - a group could only have a single user.

2. Should both migrations have a foreign key attribute?

No - only the the users table should contain a foreign key. The AddUserReferenceToGroup should be removed or you should write another migration which removes the groups.user_id column if you have pushed it to production.

3. Is setting @group.user_id = current_user.id in GroupsController#create a suitable way to assign the creating user
   to the group?

No - since the group_id column is on the users column you need to update the users table - not groups.

if @group.save
  current_user.update(group: @group)
end

4. I would also like to enforce (at the database level) that a user can only belong to one group - Is this achieved using unique => true in the schema?

No - since a row on the users table can only have one id in the groups_id column a user can only belong to one group anyways. Using unique => true would create a uniqueness index on the users.groups_id column which would only allow a single users to be associated with a group.

5. How can I enforce (at the database level) that a group must have a user?

This is not actually possible. To be able to associate a user with a group you must first insert the group into the database so that it is assigned a id.

Adding a validation on the software level would also create a "chicken vs egg" situation where a group cannot be valid since it has no users and a user cannot be associated with a group since it is not persisted.

You can however setup a constraint on the belongs_to end by declaring the foreign key column as NOT NULL.

Ruby on Rails Model method error with has_many associations(revised)

As seen in the teams controller above, the method used in the create
action was "@team = @scoreboard.teams.build(team_params)". This method
should create a has_many association between scoreboard and teams.
However, the problem is that I get an error "Undefined method "teams"
for nil class"

You haven't initialized @scoreboard, so @team = @scoreboard.teams.build(team_params) doesn't work. Defining it like @scoreboard = Scoreboard.find(params[:scoreboard_id]) should solve your problem.

Also, how do we know that teams are related to that specific
scoreboard. For example, the user could have many scoreboards. How do
we make sure that the primary key of that specific scoreboard is the
foreign key of the teams

With the foreign_key scoreboard_id available in teams table you can get the teams associated with specific scoreboard by calling @scoreboard.teams.

For example, assume you have the following code

@scoreboard = Scoreboard.find(1)
@scoreboard.teams #will return the teams associated with it.

Also I recommend you to check these guides for your better understanding on how the things work behind the scenes.

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