Relations among models enable you to query related models and perform corresponding validations.

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Individual models are easy to understand and work with. But in reality, models are often connected or related.   For applications with multiple models, you typically need to define relations between models.  

Get the app (in the state following the last article) from GitHub and install all its dependencies:

$ git clone https://github.com/strongloop/loopback-getting-started-intermediate.git
$ cd loopback-getting-started-intermediate
$ git checkout step2
$ npm install

Introducing model relations

LoopBack supports many different kinds of model relations, including: BelongsTo, HasMany, HasManyThrough, and HasAndBelongsToMany, among others. For more information, see Creating model relations.

In the Coffee Shop Reviews app, the models are related as follows:

  • A coffee shop has many reviews.
  • A coffee shop has many reviewers.
  • A review belongs to a coffee shop.
  • A review belongs to a reviewer.
  • A reviewer has many reviews.

Define relations

Now, you’re going to define these relationships between the models.  In all there are five relations.  Once again, you’ll use the lb command, but this time you’ll use the relation sub-command (relation generator).  For each relation, enter:

$ lb relation

The tool will prompt you to provide the information required to define the relation, as summarized below.

A coffee shop has many reviews; No through model and no foreign key.

? Select the model to create the relationship from: CoffeeShop
? Relation type: has many
? Choose a model to create a relationship with: Review
? Enter the property name for the relation: reviews
? Optionally enter a custom foreign key:
? Require a through model? No
? Allow the relation to be nested in REST APIs: No
? Disable the relation from being included: No

A coffee shop has many reviewers; No through model and no foreign key.

? Select the model to create the relationship from: CoffeeShop
? Relation type: has many
? Choose a model to create a relationship with: Reviewer
? Enter the property name for the relation: reviewers
? Optionally enter a custom foreign key:
? Require a through model? No
? Allow the relation to be nested in REST APIs: No
? Disable the relation from being included: No

A review belongs to a coffee shop; No foreign key.

? Select the model to create the relationship from: Review
? Relation type: belongs to
? Choose a model to create a relationship with: CoffeeShop
? Enter the property name for the relation: coffeeShop
? Optionally enter a custom foreign key:
? Allow the relation to be nested in REST APIs: No
? Disable the relation from being included: No

A review belongs to a reviewer; foreign key is publisherId.

? Select the model to create the relationship from: Review
? Relation type: belongs to
? Choose a model to create a relationship with: Reviewer
? Enter the property name for the relation: reviewer
? Optionally enter a custom foreign key: publisherId
? Allow the relation to be nested in REST APIs: No
? Disable the relation from being included: No

A reviewer has many reviews; foreign key is publisherId.

? Select the model to create the relationship from: Reviewer
? Relation type: has many
? Choose a model to create a relationship with: Review
? Enter the property name for the relation: reviews
? Optionally enter a custom foreign key: publisherId
? Require a through model? No
? Allow the relation to be nested in REST APIs: No
? Disable the relation from being included: No

Review the model JSON files

Now, look at common/models/review.json.  You should see this:

common/models/review.json

...
"relations": {
  "coffeeShop": {
    "type": "belongsTo",
    "model": "CoffeeShop",
    "foreignKey": ""
  },
  "reviewer": {
    "type": "belongsTo",
    "model": "Reviewer",
    "foreignKey": "publisherId"
  }
},
...

Likewise, common/models/reviewer.json should have this:

common/models/reviewer.json

...
"relations": {
  "reviews": {
    "type": "hasMany",
    "model": "Review",
    "foreignKey": "publisherId"
  }
},
...

And common/models/coffee-shop.json should have this:

common/models/coffee-shop.json

...
"relations": {
  "reviews": {
    "type": "hasMany",
    "model": "Review",
    "foreignKey": ""
  },
  "reviewers": {
    "type": "hasMany",
    "model": "Reviewer",
    "foreignKey": ""
  }
},
...

Next: Continue to Define access controls.

Tags: getting_started