has_many, belongs_to relation in active record mig

问题:

I have created a User model and later a Task model. I have not mentioned any relation between them while creating.

I understand that User has_many Tasks and a Task belongs_to User. I need to establish this relation between them through migration.

My question is, what would be the migration generation command for establishing that relation?

Any help would be highly appreciated.

回答1:

You could call:

rails g model task user:references

which will generates an user_id column in the tasks table and will modify the task.rb model to add a belongs_to :user relatonship. Please note, you must to put manually the has_many :tasks or has_one :task relationship to the user.rb model.

If you already have the model generated, you could create a migration with the following:

rails g migration AddUserToTask user:belongs_to

which will generate:

class AddUserToTask < ActiveRecord::Migration
  def change
    add_reference :tasks, :user, index: true
  end
end

the only difference with this approach is, the belongs_to :user relationship in the task.rb model won't be created automatically, so you must create it for your own.

回答2:

To answer the question, "What would be the migration generation command for establishing that relation?"( Meaning, how do you add a migration for existing models with a relationship like User has_many Tasks & Task belongs_to User)

The the easiest way for me to remember is like this:

>rails g migration AddUserToTask user:belongs_to

or

>rails g migration AddUserToTask user:references

:belongs_to is just an alias of :references, so either will do the same thing.

Doing it this way, the command will infer the name of the table from the migration name, set up a change method that will add the column for relationship, and configure it to be indexed:

class AddUserToTask < ActiveRecord::Migration
  def change
    add_reference :tasks, :user, index: true
  end
end

After generating that you:

>rake db:migrate

Finally, you still have to add the usual relations to your models, as is stated in the other answers, but I think this is the right answer to your question.

回答3:

How it should be done when creating the migration in the first place:

rails g scaffold child parent:references

---

What to do if you've forgotten the parent:references bit:

If you don't really have a lot defined in the model/db about the child. Your best bet might just be to run rails destroy scaffold child, and then run rails g scaffold child parent:references over it. Be sure to add the line drop_table :children if table_exists? :children before create table in the file that creates the new table. (That way if anyone pulls your code they can just run the migrations and be done.) However, it seems more probable that you will have data you don't want to lose in the child model already.

---

In that case:

rails g migration add_parent_refs_to_child

## XXXXXXXXXXXXXX_add_parent_refs_to_child.rb
class AddParentRefsToChild < ActiveRecord::Migration
  def change
    add_reference :child, :parent, index: true
  end
end

See add_reference for some more detail.

Also don't forget to make sure that the parent model has_[one | many] :children, and that the child model belongs_to :parent.

---

How not to do it:

You may be tempted just to go in and add the parent_id manually, and you certainly could, this not the best solution as it is not the conventional way to go about adding foreign keys, and doesn't lend itself very well to maintainability or readability. Convention over configuration!

The Ruby on Rails guide to association also has more helpful information on the subject.

回答4:

There is no special migration command that would be used.

In your User model you will put

class User < ActiveRecord::Base
  has_many :tasks
end

class Task < ActiveRecord::Base
  belongs_to :user
end

In the corresponding migration file for the tasks you have the following field added user_id

Take a look at this guide

回答5:

The migration will add the user's id to the task table so they know about each other

rails g migration AddUserIdToTask user_id:integer

then

rake db:migrate

And after update your controllers and views so that tasks can't be created on their own but must correspond to a user

回答6:

The Relationship in Rails is taken care by model not by Rails.

So you just need to define this relationship in your model:

class User < ActiveRecord::Base
  has_many :tasks
end

class Task < ActiveRecord::Base
  belongs_to :user
end

And just make sure that a user_id field is present in the migration for creating the "tasks" table.