I am using [ActiveAdmin][1] gem in my project.

I have 2 models using has_many through association. The database schema looks exactly the same as the example in RailsGuide. [http://guides.rubyonrails.org/association_basics.html#the-has_many-through-association][2]
[![has_many through association][3]][3]  
&lt;sub&gt;(source: [rubyonrails.org](http://guides.rubyonrails.org/images/has_many_through.png))&lt;/sub&gt;  



How can I use ActiveAdmin to ...

 1. show appointment date of each patient in physicians page?
 2. edit appointment date of each patient in physicians page?


Thanks all. :)
 

  [1]: https://activeadmin.info/
  [2]: https://guides.rubyonrails.org/association_basics.html#the-has_many-through-association
  [3]: https://i.stack.imgur.com/qaWFh.png

How to use ActiveAdmin on models using has_many through association?

I have been (hours) trouble with associations in Rails. I found a lot of similar problems, but I couldn&#39;t apply for my case:

City&#39;s class:

    class City &lt; ApplicationRecord
      has_many :users
    end

User&#39;s class:

    class User &lt; ApplicationRecord
      belongs_to :city
      
      validates :name, presence: true, length: { maximum: 80 }
      validates :city_id, presence: true
    end

Users Controller:

    def create
        Rails.logger.debug user_params.inspect
        @user = User.new(user_params)
        if @user.save!
          flash[:success] = &quot;Works!&quot;
          redirect_to &#39;/index&#39;
        else
          render &#39;new&#39;
        end
     end

    def user_params
      params.require(:user).permit(:name, :citys_id)
    end

Users View:

    &lt;%= form_for(:user, url: &#39;/user/new&#39;) do |f| %&gt;
      &lt;%= render &#39;shared/error_messages&#39; %&gt;

      &lt;%= f.label :name %&gt;
      &lt;%= f.text_field :name %&gt;
      
      &lt;%= f.label :citys_id, &quot;City&quot; %&gt;
      &lt;select name=&quot;city&quot;&gt;
        &lt;% @city.all.each do |t| %&gt;
          &lt;option value=&quot;&lt;%= t.id %&gt;&quot;&gt;&lt;%= t.city %&gt;&lt;/option&gt;
        &lt;% end %&gt;
      &lt;/select&gt;
    end

Migrate:

    class CreateUser &lt; ActiveRecord::Migration[5.0]
      def change
        create_table :user do |t|
          t.string :name, limit: 80, null: false
          t.belongs_to :citys, null: false
          t.timestamps
      end
    end

Message from console and browser:

    ActiveRecord::RecordInvalid (Validation failed: City must exist):


Well, the problem is, the attributes from User&#39;s model that aren&#39;t FK they are accept by User.save method, and the FK attributes like citys_id are not. Then it gives me error message in browser saying that &quot;Validation failed City must exist&quot;.

Thanks



Validation failed Class must exist

So I know how to override the default getters for attributes of an ActiveRecord object using

    def custom_getter
      return self[:custom_getter] || some_default_value
    end

I&#39;m trying to achieve the same thing however for a belongs to association.  For instance.

    class Foo &lt; AR
      belongs_to :bar

      def bar
        return self[:bar] || Bar.last
      end
    end
    
    class Bar &lt; AR
      has_one :foo
    end

When I say:

    f = Foo.last

I&#39;d like to have the method `f.bar` return the last Bar, rather than nil if that association doesn&#39;t exist yet.

This doesn&#39;t work however.  The reason is that self[:bar] is always undefined.  It&#39;s actually self[:bar_id].

I can do something naive like:

    def bar
      if self[:bar_id]
        return Bar.find(self[:bar_id])
      else
        return Bar.last
      end
    end

However this will always make a db call, even if Bar has already been fetched, which is certainly not ideal.

Does anyone have an insight as to how I might have a relationship such that the belongs_to attribute is only loaded once and has a default value if not set.


rails override default getter for a relationship (belongs_to)

I have a one to many association between jobs and companies and it works fine. In the job form view I have text_field for the company name with an autocomplete feature. The autocomplete works fine but the find_or_create_by don&#39;t create a new company if I put a company name that doesn&#39;t exist in the autocomplete list.

      def company_name
        company.try(:name)
      end
      
      def company_name=(name)
        @company = Company.find_or_create_by(name: name)
      end

Rails 4 find or create by method doesn&#39;t work

Here I am, with another question about aggregation and association. I wanted to learn some basics of UML, so I started reading &quot;UML distilled&quot; by Martin Fowler. I read both chapters about classes, and there is one thing that I can&#39;t fully grasp I think, and that is aggregation vs association. In the book there is this quote:

&gt; In the pre-UML days, people were usually rather vague on what was aggregation and what was
association. Whether vague or not, they were always inconsistent with everyone else. As a result,
many modelers think that aggregation is important, although for different reasons. So  the UML
included aggregation (Figure 5.3) but with hardly any semantics. As Jim Rumbaugh says, &quot;Think of it
as a modeling placebo&quot; *[Rumbaugh, UML Reference].*

As I understand from this quote and topics that I read on Stack Overflow it doesn&#39;t matter which one of those two relations I use, they mean basically the same, or is there any situation where the usage of aggregation instead of association would be justified and/or I could not change one to the other without changing the &quot;meaning&quot; of a class diagram?

I am asking this, because this book is from 2003 and some things could have changed during those few years.


UML aggregation vs association

I&#39;ve got myself a question regarding associations in Sails.js version 0.10-rc5. I&#39;ve been building an app in which multiple models are associated to one another, and I&#39;ve arrived at a point where I need to get to nest associations somehow.

There&#39;s three parts:

First there&#39;s something like a blog post, that&#39;s being written by a user. In the blog post I want to show the associated user&#39;s information like their username. Now, everything works fine here. Until the next step: I&#39;m trying to show comments which are associated with the post.

The comments are a separate Model, called Comment. Each of which also has an author (user) associated with it. I can easily show a list of the Comments, although when I want to display the User&#39;s information associated with the comment, I can&#39;t figure out how to populate the Comment with the user&#39;s information.

In my controller i&#39;m trying to do something like this:

    Post
      .findOne(req.param(&#39;id&#39;))
      .populate(&#39;user&#39;)
      .populate(&#39;comments&#39;) // I want to populate this comment with .populate(&#39;user&#39;) or something
      .exec(function(err, post) {
        // Handle errors &amp; render view etc.
      });

In my Post&#39;s &#39;show&#39; action i&#39;m trying to retrieve the information like this (simplified):

    &lt;ul&gt; 
      &lt;%- _.each(post.comments, function(comment) { %&gt;
        &lt;li&gt;
          &lt;%= comment.user.name %&gt;
          &lt;%= comment.description %&gt;
        &lt;/li&gt;
      &lt;% }); %&gt;
    &lt;/ul&gt;

The comment.user.name will be undefined though. If I try to just access the &#39;user&#39; property, like comment.user, it&#39;ll show it&#39;s ID. Which tells me it&#39;s not automatically populating the user&#39;s information to the comment when I associate the comment with another model. 

Anyone any ideals to solve this properly :)?

Thanks in advance!


P.S.

For clarification, this is how i&#39;ve basically set up the associations in different models:

    // User.js
    posts: {
      collection: &#39;post&#39;
    },   
    hours: {
      collection: &#39;hour&#39;
    },
    comments: {
      collection: &#39;comment&#39;
    }
    
    // Post.js
    user: {
      model: &#39;user&#39;
    },
    comments: {
      collection: &#39;comment&#39;,
      via: &#39;post&#39;
    }
    
    // Comment.js
    user: {
      model: &#39;user&#39;
    },
    post: {
      model: &#39;post&#39;
    }


Sails.js populate nested associations

I have looked through the Ruby on Rails guides and I can&#39;t seem to figure out how to prevent someone from deleting a Parent record if it has Children.  For example.  If my database has **CUSTOMERS** and each customer can have multiple **ORDERS**, I want to prevent someone from deleting a customer if it has any orders in the database.  They should only be able to delete a customer if it has no orders.

Is there a way when defining the association between models to enforce this behavior?

How do I prevent deletion of parent if it has child records?

This follows [this][1] prior question, which was answered. I actually discovered I could remove a join from that query, so now the working query is

    start_cards = DeckCard.find :all, :joins =&gt; [:card], :conditions =&gt; [&quot;deck_cards.deck_id = ? and cards.start_card = ?&quot;, @game.deck.id, true]  

This appears to work. However, when I try to move these DeckCards into another association, I get the ActiveRecord::ReadOnlyRecord error.

Here&#39;s the code

    for player in @game.players 
      player.tableau = Tableau.new
      start_card = start_cards.pop 
      start_card.draw_pile = false
      player.tableau.deck_cards &lt;&lt; start_card  # the error occurs on this line
    end

and the relevant Models (tableau are the players cards on the table)

    class Player &lt; ActiveRecord::Base
      belongs_to :game
      belongs_to :user
      has_one :hand
      has_one :tableau
    end

    class Tableau &lt; ActiveRecord::Base
      belongs_to :player
      has_many :deck_cards
    end  

    class DeckCard &lt; ActiveRecord::Base
      belongs_to :card
      belongs_to :deck  
    end

I am doing a similar action just after this code, adding `DeckCards` to the players hand, and that code is working fine. I wondered if I needed `belongs_to :tableau` in the DeckCard Model, but it works fine for the adding to player&#39;s hand. I do have a `tableau_id` and `hand_id` columns in the DeckCard table.

I looked up ReadOnlyRecord in the rails api, and it doesn&#39;t say much beyond the description.

  [1]: https://stackoverflow.com/questions/628000/rails-whats-wrong-with-this-multiple-join-with-conditions-on-the-associations &quot;this&quot;

What is causing this ActiveRecord::ReadOnlyRecord error?

In rails guides it&#39;s described like this:

&gt;Objects will be in addition destroyed if they’re associated with `:dependent =&gt; :destroy`, and deleted if they’re associated with `:dependent =&gt; :delete_all`

Right, cool. But what&#39;s the difference between being destroyed and being deleted?
I tried both and it seems to do the same thing.

Rails :dependent =&gt; :destroy VS :dependent =&gt; :delete_all

What is the difference between Unidirectional and Bidirectional associations?

Since the table generated in the db are all the same,so the only difference I found is that each side of the bidiretional assocations will have a refer to the other,and the unidirectional not.


**This is a Unidirectional association**

    public class User {
    	private int		id;
    	private String	name;
    	@ManyToOne
    	@JoinColumn(
    			name = &quot;groupId&quot;)
    	private Group	group;
    }
    
    public class Group {
    	private int		id;
    	private String	name;
    }
    
**The Bidirectional association**
    
    public class User {
    	private int		id;
    	private String	name;
    	@ManyToOne
    	@JoinColumn(
    			name = &quot;groupId&quot;)
    	private Group	group;
    }
    public class Group {
    	private int			id;
    	private String		name;
    	@OneToMany(mappedBy=&quot;group&quot;)
    	private List&lt;User&gt;	users;
    }


The difference is whether the group holds a reference of the user.

So I wonder if this is the only difference? which is recommended?

What is the difference between Unidirectional and Bidirectional JPA and Hibernate associations?

How would you do a many-to-many association with MongoDB?

For example; let&#39;s say you have a Users table and a Roles table. Users have many roles, and roles have many users. In SQL land you would create a UserRoles table.

    Users:
        Id
        Name

    Roles:
        Id
        Name

    UserRoles:
        UserId
        RoleId

How is same sort of relationship handled in MongoDB?


MongoDB Many-to-Many Association

In this example, I create a `user` with no `profile`, then later on create a `profile` for that user. I tried using build with a `has_one` association but that blew up. The only way I see this working is using `has_many`. The `user` is supposed to only have at most one `profile`.

I have been trying this. I have:

    class User &lt; ActiveRecord::Base
      has_one :profile
    end
    
    class Profile &lt; ActiveRecord::Base
      belongs_to :user
    end

But when I do:

    user.build_profile 

I get the error:

    ActiveRecord::StatementInvalid: Mysql::Error: Unknown column &#39;profiles.user_id&#39; in &#39;where clause&#39;: SELECT * FROM `profiles` WHERE (`profiles`.user_id = 4)  LIMIT 1

Is there a way in rails to have 0 or 1 association? 

Rails: Using build with a has_one association in rails

So I have a create_table like this for Courses at a School:

    create_table :courses do |t|
      t.string :name
      t.references :course
      t.timestamps
    end

but I want it to reference **two** other courses like:

    has_many :transferrable_as # A Course
    has_many :same_as          # Another Course

Can I say the following? 

    t.references :transferrable_as, :as=&gt; :course


Rails migration: t.references with alternative name?

In the Rails 3 [docs][1], the `build` method for associations is described as being the same as the `new` method, but with the automatic assignment of the foreign key. Straight from the docs:

    Firm#clients.build (similar to Client.new(&quot;firm_id&quot; =&gt; id))

  [1]: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

I&#39;ve read similar elsewhere.

However, when I use `new` (e.g. `some_firm.clients.new` without any parameters), the new client&#39;s `firm_id` association **is** automatically created. I&#39;m staring at the results right now in the console!

Am I missing something? Are the docs a bit out of date (unlikely)? What&#39;s the difference between `build` and `new`?

Build vs new in Rails 3

Given the following

    class User &lt; ActiveRecord::Base
      has_and_belongs_to_many :companies
    end
    
    class Company &lt; ActiveRecord::Base
      has_and_belongs_to_many :users
    end

how do you define factories for companies and users including the bidirectional association? Here&#39;s my attempt

    Factory.define :company do |f|
      f.users{ |users| [users.association :company]}
    end
    
    Factory.define :user do |f|
      f.companies{ |companies| [companies.association :user]}
    end

now I try 

    Factory :user

Perhaps unsurprisingly this results in an infinite loop as the factories recursively use each other to define themselves.

More surprisingly I haven&#39;t found a mention of how to do this anywhere, is there a pattern for defining the necessary factories or I am doing something fundamentally wrong? 



How to create has_and_belongs_to_many associations in Factory girl

What is the difference between association, aggregation, and composition? 
Please explain in terms of implementation.

What is the difference between association, aggregation and composition?

I want to be able to use two columns on one table to define a relationship. So using a task app as an example. 

**Attempt 1:**

    class User &lt; ActiveRecord::Base
      has_many :tasks
    end

    class Task &lt; ActiveRecord::Base
      belongs_to :owner, class_name: &quot;User&quot;, foreign_key: &quot;owner_id&quot;
      belongs_to :assignee, class_name: &quot;User&quot;, foreign_key: &quot;assignee_id&quot;
    end

So then `Task.create(owner_id:1, assignee_id: 2)`

This allows me to perform `Task.first.owner` which returns *user one* and `Task.first.assignee` which returns *user two* but `User.first.task` returns nothing. Which is because task doesn&#39;t belong to a user, they belong to *owner* and *assignee*. So,

**Attempt 2:**


    class User &lt; ActiveRecord::Base
      has_many :tasks, foreign_key: [:owner_id, :assignee_id]
    end

    class Task &lt; ActiveRecord::Base
      belongs_to :user
    end

That just fails altogether as two foreign keys don&#39;t seem to be supported. 

So what I want is to be able to say `User.tasks` and get both the users owned and assigned tasks. 

Basically somehow build a relationship that would equal a query of `Task.where(owner_id || assignee_id == 1)`

Is that possible?

# Update
I&#39;m not looking to use `finder_sql`, but this issue&#39;s unaccepted answer looks to be close to what I want: https://stackoverflow.com/questions/3421873/rails-multiple-index-key-association

So this method would look like this,

**Attempt 3:**

    class Task &lt; ActiveRecord::Base
      def self.by_person(person)
        where(&quot;assignee_id =&gt; :person_id OR owner_id =&gt; :person_id&quot;, :person_id =&gt; person.id
      end 
    end

    class Person &lt; ActiveRecord::Base

      def tasks
        Task.by_person(self)
      end 
    end

Though I can get it to work in `Rails 4`, I keep getting the following error:

    ActiveRecord::PreparedStatementInvalid: missing value for :owner_id in :donor_id =&gt; :person_id OR assignee_id =&gt; :person_id

Rails association with multiple foreign keys

What&#39;s the best practice to create has_one relations?

For example, if I have a user model, and it must have a profile...

How could I accomplish that? 

One solution would be:

    # user.rb
    class User &lt;&lt; ActiveRecord::Base
      after_create :set_default_association

      def set_default_association
        self.create_profile
      end
    end

But that doesn&#39;t seem very clean...  Any suggestions? 

Rails - Best-Practice: How to create dependent has_one relations

Rails has a `has_one :through` association that helps set up a one-to-one association with a third model by going through a second model. What is the real use of that besides making a shortcut association, that would otherwise be an extra step away.

Taking this example from the Rails [guide][1]:

    class Supplier &lt; ActiveRecord::Base
      has_one :account
      has_one :account_history, :through =&gt; :account
    end
    
    class Account &lt; ActiveRecord::Base
      belongs_to :supplier
      has_one :account_history
    end
    
    class AccountHistory &lt; ActiveRecord::Base
      belongs_to :account
    end

might allow us to do something like:

    supplier.account_history

which would otherwise be reached as:

    supplier.account.history

If it&#39;s only for simpler access then technically there could be a one-to-one association that connects a model with some nth model going through n-1 models for easier access. Is there anything else to it that I am missing besides the shortcut? 

  [1]: http://guides.rubyonrails.org/association_basics.html

Rails has_one :through association

I have tons of instances where I need to implement some sort of Polymorphic Association in my database. I always waste tons of time thinking through all the options all over again. Here are the 3 I can think of. I&#39;m hoping there is a best practice for SQL Server.

Here is the multiple column approach

![Multiple Column approach][1]


Here is the no foreign key approach

![No Foreign Key Approach][2]


And here is the base table approach

![Base table approach][3]


  [1]: http://i.stack.imgur.com/BH1LP.jpg
  [2]: http://i.stack.imgur.com/TQa7C.jpg
  [3]: http://i.stack.imgur.com/7jFE5.jpg

What is the best way to implement Polymorphic Association in SQL Server?

I have 2 models. `Report` and `Server` that have a belongs_to and has_many relationship. I created an accessor method using `delegate` that allows a `Report` to find its associated `Server.company_id`. Now, I want to run a query on `Report` that allows me to find all `Report` that are associated with a specific `Server` that has a specific `company_id` attribute of 5.

Here are my two models. And yes I know the current query wont work since `Report` does not have an attribute `company_id`.

And no, I dont want to store `company_id` inside of `Report` since that information doesn&#39;t belong in `Report`.


**Report**

    class Report &lt; ActiveRecord::Base
      
     belongs_to :server
    
     delegate :company_id, :to =&gt; :server
    
     	class &lt;&lt; self
    
    	  	def method(url, base_url)
    			#Report.where(company_id: 5)
    	  	end
    	end
    
    end

**Server**

    class Server &lt; ActiveRecord::Base
    
     attr_accessible :company_id
      
     has_many :reports
    
    end





Rails: ActiveRecord query based on association value

I&#39;m wondering how I can add associations to my models. Suppose, I generate two models

    rails generate model User
    rails generate model Car

Now I want to add an associations so that the models acquire the form

    class User &lt; ActiveRecord::Base
      has_many :cars
    end
    class Car &lt; ActiveRecord::Base
      belongs_to :user
    end
The question is: how to apply this modification by migrations in order to obtain cars_users table in the database? I&#39;m planning to use that table in my code. 


add associations to exisiting models

Is it possible to have multiple `has_many :through` relationships that pass through each other in Rails?  I received the suggestion to do so as a solution for another question I posted, but have been unable to get it to work.

Friends are a **cyclic association** through a join table.  The goal is to create a `has_many :through` for `friends_comments`, so I can take a `User` and do something like `user.friends_comments` to get all comments made by his friends in a single query.

    class User
      has_many :friendships
      has_many :friends, 
               :through =&gt; :friendships,
               :conditions =&gt; &quot;status = #{Friendship::FULL}&quot;
      has_many :comments
      has_many :friends_comments, :through =&gt; :friends, :source =&gt; :comments
    end

    class Friendship &lt; ActiveRecord::Base
      belongs_to :user
      belongs_to :friend, :class_name =&gt; &quot;User&quot;, :foreign_key =&gt; &quot;friend_id&quot;
    end

This looks great, and makes sense, but isn&#39;t working for me.  This is the error I&#39;m getting in relevant part when I try to access a user&#39;s friends_comments:  
`ERROR:  column users.user_id does not exist  
: SELECT &quot;comments&quot;.* FROM &quot;comments&quot;  INNER JOIN &quot;users&quot; ON &quot;comments&quot;.user_id = &quot;users&quot;.id    WHERE ((&quot;users&quot;.user_id = 1) AND ((status = 2)))`

When I just enter user.friends, which works, this is the query it executes:  
`: SELECT &quot;users&quot;.* FROM &quot;users&quot; INNER JOIN &quot;friendships&quot; ON &quot;users&quot;.id = &quot;friendships&quot;.friend_id WHERE ((&quot;friendships&quot;.user_id = 1) AND ((status = 2)))`

So it seems like it&#39;s entirely forgetting about the original `has_many` through friendship relationship, and then is inappropriately trying to use the User class as a join table.

Am I doing something wrong, or is this simply not possible?

All Associations Solutions on Gang of Coders

Total of 23 Associations Solutions