Write a Migration with Reference to a Model Twice

Write a migration with reference to a model twice

In the migration, create two different columns for each kind of user. For example:

add_column :messages, :sender_id, :integer
add_column :messages, :receiver_id, :integer

Then in the model, that's where the logic to map each column to the User class happens:

belongs_to :sender, :class_name => 'User'
belongs_to :receiver, :class_name => 'User'

Of course, use your own words for sender and receiver, but Rails will automatically associate sender to the sender_id column (and the same logic for receiver)

You will then be able to interact with both user user.sender and user.receiver.

How do I add migration with multiple references to the same model in one table? Ruby/Rails

You can do this simply with the add_column method in your migrations and set up the proper associations in your classes:

class AddFields < ActiveRecord::Migration
  def change
    add_column :tickets, :image_1_id, :integer
    add_column :tickets, :image_2_id, :integer
  end
end

class Ticket < ActiveRecord::Base
  belongs_to :image_1, :class_name => "Image"
  belongs_to :image_2, :class_name => "Image"
end

class Image < ActiveRecord::Base
  has_many :primary_tickets, :class_name => "Ticket", :foreign_key => "image_1_id"
  has_many :secondary_tickets, :class_name => "Ticket", :foreign_key => "image_2_id"
end

This blog post, Creating Multiple Associations with the Same Table, goes into more detail.

Ruby on rails - Reference the same model twice?

Add this to your Model

belongs_to :sender, :class_name => "User"
belongs_to :recipient, :class_name => "User"

And you are able to call @message.sender and @message.recipient and both reference to the User model.

Instead of user:references in your generate command, you'd need sender:references and recipient:references

Referencing the same model twice in Rails with a simple entity?

You can't actually use a single assocation here as ActiveRecord does not support assocations where the foreign key could be in one of two columns. Each has_one/has_many corresponds to a single foreign key column on the inverse side.

Instead you need one assocation on Address for each foreign key on Shipment:

class Shipment < ApplicationRecord
  belongs_to :address_from, 
     class_name: 'Address',
     inverse_of: :outgoing_shipments
  belongs_to :address_to, 
     class_name: 'Address',
     inverse_of: :incoming_shipments
end

class Address < ApplicationRecord
  has_many :outgoing_shipments,
    class_name: 'Shipment',
    foreign_key: :address_from_id,
    inverse_of: :address_from
  has_many :incoming_shipments,
    class_name: 'Shipment',
    foreign_key: :address_to_id,
    inverse_of: :address_to
end

While you could use has_one here you should note that there is nothing preventing a address from having multiple shipments unless you add uniqueness constraints on shipments.address_from_id and shipments.address_to_id and validations. Not sure why you would want this though.

Referencing a table twice in rails/postgres

This is due to PostgreSql not understanding how to make the custom association foreign keys.

Try changing the migration file to,

class CreateTransactions < ActiveRecord::Migration
  def change
    create_table :transactions do |t|
      t.integer :buyer_id
      t.integer :seller_id
      t.integer :product_id

      t.timestamps null: false
    end
    add_index(:transactions, :buyer_id)
    add_index(:transactions, :seller_id)
    add_index(:transactions, :product_id)
    add_foreign_key :transactions, :users, column: :buyer_id
    add_foreign_key :transactions, :users, column: :seller_id
  end
end

It automatically links buyer_id to user, taking the power of

belongs_to :buyer, :class_name => 'User'

The above line makes the buyer_id as foreign key.

You can also try,

class CreateTransactions < ActiveRecord::Migration
  def change
    create_table :transactions do |t|
      t.references :buyer, index: true
      t.references :seller, index: true
      t.references :product, index: true
      t.timestamps null: false
    end
    add_foreign_key :transactions, :users, column: :buyer_id
    add_foreign_key :transactions, :users, column: :seller_id
  end
end

Creating Knex Migration Referencing the Same Table Twice

For anyone who is interested... Consolidating the working solution above using Node/ExpressJS for the server, Postgresql for the db, KnexJS/ObjectiveJS to manage Models and queries.

Here is the migration that sets up columns for buyers and sellers both of which reference the same Users table:

exports.up = function(knex, Promise) {
  return knex.schema.createTable('likes', t => {
    t.increments('id').primary()
    t.integer('buyers_id').references('users.id').onDelete('CASCADE')
    t.integer('sellers_id').references('users.id').onDelete('CASCADE')

    ...

    t.datetime("created_at");
    t.datetime("updated_at");
  })
};

HEre is the Transactions Model including associations s.t. a Transaction belongs to Item Buyer(user) and Seller(user) and Item:

const BaseModel = require("./BaseModel");

// const Password = require('objection-password')();

class Transaction extends BaseModel {
  static get tableName() {
    return "transactions";
  }

  static get relationMappings () {
    const User = require('./User');
    const Item = require('./Item')
    return {
      buyer: {
        relation: BaseModel.BelongsToOneRelation,
        modelClass: User,
        join: {
          from: 'transactions.buyers_id',
          to: 'users.id'
        }
      },
      seller: {
        relation: BaseModel.BelongsToOneRelation,
        modelClass: User,
        join: {
          from: 'transactions.sellers_id',
          to: 'users.id'
        }
      }, 
      books: {
        relation: BaseModel.BelongsToOneRelation,
        modelClass: Item,
        join: {
          from: 'transactions.items_id',
          to: 'items.id'
        }
      }
    }
  }
}

module.exports = Transaction;

Lastly, here is the express route which returns all transactions including eager loading the associated models:

let router = express.Router();

router.get('/', async (req, res) => {
  const transactions = await Transaction
    .query()
    .eager(['buyer', 'items')
  res.json(transactions);
});

rails belongs_to same model twice

Something like this:

class Task < ActiveRecord::Base
  belongs_to :soldier
  belongs_to :creator, class_name: Soldier, foreign_key: :created_by
end

class Soldier < ActiveRecord::Base
  has_many :tasks

  # Optional -- I'm unclear if you want this too?
  has_many :created_tasks, class_name: Task, foreign_key: :created_by
end

The above implementation is OK. However, if you wish to stick within rails conventions (i.e. not need to manually specify a foreign_key), then you could call the database column creator_id instead of created_by.

How do I create a code first model that references another model twice in C# MVC?

This error usually spawns when you are trying to rename table.

I have a suggestion for you to create your Model and try to achieve your desired results with minimal possible fields. Also, you want to use virtual keyword only on foreign entities.

I've just tested something similar (Admin, AdminLog[Contains Admin1 and Admin2]) and it worked just fine.

Code you should try

public class System
{
    public int ID { get; set; }
    public string Name { get; set; }
}

public class Jumplane
{
    public int ID { get; set; }
    public virtual System System1 { get; set; }
    public virtual System System2 { get; set; }
}

Now that this code will work and you will be able to execute your desired LINQ query just add all other fields to it.

Some pointers (maybe subjective)

• Virtual for foreign only
• Classes start with capital letter
• No need for SystemID, SystemName fields, just ID and Name will do since you will see that it belongs to System

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