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Objectives

1. Primary Key
2. Rails Migration Foreign Key
3. Foreign Key Rails
4. Generate Model With Foreign Key Rails For Kids
5. Rails Generate Model With Foreign Key

Rails 5.1 + Using a UUID as a primary key in ActiveRecord with PostgreSQL. Activerecord, primarykeytype. Now when you run rails generate model post. Adds a new foreign key. Fromtable is the table with the key column, totable contains the referenced primary key. The foreign key will be named after the following pattern: fkrails.identifier is a 10 character long string which is deterministically generated from the fromtable and column.A custom name can be specified with the:name option. In both cases, the second model contains one reference to the first model in form of a foreign key. The second model doesn't know about the first model relation to it - it is not aware if the first model has a reference to more than one of them or just to one. The tests for associations get more complex to write the more relations you create. Feb 01, 2016 Watch how to run the Rails scaffold generator and include references to other models using the 'references' association method. Understanding Model in Ruby on Rails. Creating simple model. By convention, Rails guesses that the column in the join table used to hold the foreign key pointing to this model is the name of this model with the suffix id added. The:foreignkey option lets you set the name of the foreign key directly. To create a completely brand new reference with a foreign key(in Rails 4.2), generate a migration using the following command: which will create a migration file as: This will add a new foreign key to the userid column of the uploads table. The key references the id column in users table.

After this lesson, developers will be able to:

• Create a one-to-many relationship between 2 models
• Perform CRUD actions on two models using rails console
• Describe primary and foreign keys

Preparation

Before this lesson, developers should already be able to:

• Access and use rails console
• Perform CRUD actions on one model

Framing: Review One-to-Many Relationships

Previously, we created a Rails app using an Artist as our model. In this lesson, we're gonna add a Song model that will belong to the Artist. Since an Artist typically has many songs we're gonna create a One to Many Relationship between the two models.

YOU DO- What are some other examples of one-to-many relationships that you can think of?

• A Post has many Comments
• A Album has many Songs
• A Movie has many Actors

In our Muse app, an Artist will have many instances of Song.We need a way to represent this type of many to many relationship!

Primary and Foreign keys

Here is an example of a one-to-many relationship from the Rails Docs.

Note that both authors and books each have their own id field. These are primary keys.

Now, how can we relate the books that belong to a given author? We add an author_id field, or foreign key, to the books table. The author_id will be the id of the author who wrote the book.

Generating the Model / Migration

We can generate the Song model just like our Artist model! If we specify the attributes (i.e.columns on the command line, Rails will automatically generate the correct migration for us.)

YOU DO - Based on what we covered, how can we create an Artist model.. what would be the rails command to create a Song model with title and genre attributes?

1. From the command line, generate the Song model:

   • We'll see shortly that artist:references will add a foreign key field of artist_id to our songs table

2. Run rails db:migrate. This will generate a Song model, with artist_id, title, and genre columns. We can look at db/schema.rb file to confirm.

Adding the Active Record Relationships

We need to update our models to indicate the associations between them. For Muse, our models should look like so:

Adding has_many :songs will complete the one-to-many relationship.

Dependent Destroy

What happens if we delete an Artist? The songs remain. This doesn't make sense right? If we delete an Artist then their Songs should be deleted too? Rails has a way to add this functionality:

Seed some Songs

Cool, like before, let me give you some song data to seed your database with songs:

1. Add this below your Artist seeds in db/seeds.rb:

2. We don't want to re-seed the duplicate artists from earlier. So let's drop our database and start over.

   rails db:drop db:create db:migrate db:seed

   Not Recommended: We can run rails db:reset(for drop and create only)

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3. Cool, let's go into rails c and make sure that our database has our Songs:

Primary Key

CRUD: Let's try it on our Song model

Create

.new + .save

The cool thing about Active Record relationships is that now we can create a new Song directly through the Artist it belongs to:

To save our instance to the database we use .save:

Create a Song for an Artist of your choice using .new and .save.

Rails Migration Foreign Key

.create

The create method will both instantiate and save a new record into the database:

The coolest part of this is that Active Record will add the appropriate foreign keyartist_id field into the Song.

Also, what if you don't know the id of the Artist?

Foreign Key Rails

Add a Song to your Artist using .create

Read

Or, what if we know the song title and want to find out who the Artist is?

Note.. when we want to return all the artists or songs we use uppercase, as in Song.all. This is because we're asking for all the instances of the Song model/class.

When we want to access the songs for a given artist we use lowercase and plural, as in rihanna.songs.

• Find the Artist that sings 'Shake It Off'

You can learn more about querying an Active Record model in the Active RecordQuery Interface guide.

Update

• Choose a Artist and update the attributes of a Song

Delete

Likewise, once retrieved, an Active Record object can be destroyed which removes it from the database.

Let's delete a Song:

Generate Model With Foreign Key Rails For Kids

Remember the code dependent: :destroy that we added to the Artist model? That bit of code tells Active Record that if we delete an Artist it should also delete all the songs that belong to that Artist.

Let's see it in action. When we delete Rihanna all of her songs are deleted also.

Check out your rails c and note that Active Record is deleting both the Artist and her Songs.

• delete an Artist

Independent Practice (15 mins)

Using Active Record Associations:

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• Create 3 new Songs for an Artist of your choice
• Update a song's attribute
• Delete a song you created

Conclusion (5 mins)

In this lesson we:

Rails Generate Model With Foreign Key

• Created a Song model for our Muse app
• Established a one-to-many relationship between Artist and Song
• Practiced CRUD with Active Record associations

Labtime

Add onto the books-app you started, that was based on the previous lesson. Go through this lesson and instead of Song add a Book model to your app.