@loopback/docs

--- lang: en title: 'Model' keywords: LoopBack 4.0, LoopBack 4 sidebar: lb4_sidebar permalink: /doc/en/lb4/Model.html --- ## Overview A model describes business domain objects, for example, `Customer`, `Address`, and `Order`. It usually defines a list of properties with name, type, and other constraints. {% include note.html content="Models describe the shape of data. Behavior like CRUD operations is added by repositories. This is different from LoopBack 3.x where models implement behavior too." %} {% include tip.html content="A single model can be used with multiple different Repositories." %} Models can be used for data exchange on the wire or between different systems. For example, a JSON object conforming to the `Customer` model definition can be passed in REST/HTTP payload to create a new customer or stored in a document database such as MongoDB. Model definitions can also be mapped to other forms, such as relational database schemas, XML schemas, JSON schemas, OpenAPI schemas, or gRPC message definitions, and vice versa. There are two subtly different types of models for domain objects: - Value Object: A domain object that does not have an identity (ID). Its equality is based on the structural value. For example, `Address` can be modeled as a `Value Object` because two US addresses are equal if they have the same street number, street name, city, and zip code values. For example: ```json { "name": "Address", "properties": { "streetNum": "string", "streetName": "string", "city": "string", "zipCode": "string" } } ``` - Entity: A domain object that has an identity (ID). Its equality is based on the identity. For example, `Customer` can be modeled as an `Entity` because each customer has a unique customer id. Two instances of `Customer` with the same customer id are equal since they refer to the same customer. For example: ```json { "name": "Customer", "properties": { "id": "string", "lastName": "string", "firstName": "string", "email": "string", "address": "Address" } } ``` Currently, we provide the `@loopback/repository` module, which provides special decorators for adding metadata to your TypeScript/JavaScript classes in order to use them with the legacy implementation of the [datasource juggler](https://github.com/strongloop/loopback-datasource-juggler). ## Definition of a Model At its core, a model in LoopBack is a simple JavaScript class. ```ts export class Customer { email: string; isMember: boolean; cart: ShoppingCart; } ``` Extensibility is a core feature of LoopBack. There are external packages that add additional features, for example, integration with the juggler bridge or JSON Schema generation. These features become available to a LoopBack model through the `@model` and `@property` decorators from the `@loopback/repository` module. ```ts import {model, property} from '@loopback/repository'; @model() export class Customer { @property() email: string; @property() isMember: boolean; @property() cart: ShoppingCart; } ``` ## Model Discovery LoopBack can automatically create model definitions by discovering the schema of your database. See [Discovering models](Discovering-models.md) for more details and a list of connectors supporting model discovery. ## Using the Juggler Bridge To define a model for use with the juggler bridge, extend your classes from `Entity` and decorate them with the `@model` and `@property` decorators. ```ts import {model, property, Entity} from '@loopback/repository'; @model() export class Product extends Entity { @property({ id: true, description: 'The unique identifier for a product', }) id: number; @property() name: string; @property() slug: string; constructor(data?: Partial<Product>) { super(data); } } ``` Models are defined primarily by their TypeScript class. By default, classes forbid additional properties that are not specified in the type definition. The persistence layer respects this constraint and configures underlying PersistedModel classes to enforce `strict` mode. To create a model that allows both well-defined but also arbitrary extra properties, you need to disable `strict` mode in model settings through the CLI and tell TypeScript to allow arbitrary additional properties to be set on model instances. ```ts @model({settings: {strict: false}}) class MyFlexibleModel extends Entity { @property({id: true}) id: number; // Define well-known properties here // Add an indexer property to allow additional data [prop: string]: any; } ``` ### Model Decorator The model decorator can be used without any additional parameters, or can be passed in a  [ModelDefinitionSyntax](https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html) object which follows the general format provided in LoopBack 3: ```ts @model({ name: 'Category', properties: { // define properties here. }, settings: { // etc... }, }) class Category extends Entity { // etc... } ``` However, the model decorator already knows the name of your model class, so you can omit it. ```ts @model() class Product extends Entity { name: string; // other properties... } ``` Additionally, the model decorator is able to build the properties object through the information passed in or inferred by the property decorators, so the properties key value pair can also be omitted. ### Property Decorator The property decorator takes in the same arguments used in LoopBack 3 for individual property entries: ```ts @model() class Product extends Entity { @property({ name: 'name', description: "The product's common name.", type: 'string', }) public name: string; } ``` The complete list of valid attributes for property definitions can be found in LoopBack 3's [Model definition section](https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html#properties). You can also specify the validation rules in the field `jsonSchema`. For example: ```ts @model() class Product extends Entity { @property({ name: 'name', description: "The product's common name.", type: 'string', // Specify the JSON validation rules here jsonSchema: { maxLength: 30, minLength: 10, }, }) public name: string; } ``` Check out the documentation of [Parsing requests](Parsing-requests.md#request-body) to see how to do it in details.  The property decorator leverages LoopBack's [metadata package](https://github.com/strongloop/loopback-next/tree/master/packages/metadata) to determine the type of a particular property. ```ts @model() class Product extends Entity { @property() public name: string; // The type information for this property is String. } ``` ### Array Property Decorator There is a limitation to the metadata that can be automatically inferred by LoopBack, due to the nature of arrays in JavaScript. In JavaScript, arrays do not possess any information about the types of their members. By traversing an array, you can inspect the members of an array to determine if they are of a primitive type (string, number, array, boolean), object or function, but this does not tell you anything about what the value would be if it were an object or function. For consistency, we require the use of the `@property.array` decorator, which adds the appropriate metadata for type inference of your array properties. ```ts @model() class Order extends Entity { @property.array(Product) items: Product[]; } @model() class Thread extends Entity { // Note that we still require it, even for primitive types! @property.array(String) posts: string[]; } ``` Additionally, the `@property.array` decorator can still take an optional second parameter to define or override metadata in the same fashion as the `@property` decorator. ```ts @model() class Customer extends Entity { @property.array(String, { name: 'names', required: true, }) aliases: string[]; } ``` ### JSON Schema inference Use the `@loopback/repository-json-schema module` to build a JSON schema from a decorated model. Type information is inferred from the `@model` and `@property` decorators. The `@loopback/repository-json-schema` module contains the `getJsonSchema` function to access the metadata stored by the decorators to build a matching JSON schema of your model. ```ts import {model, property} from '@loopback/repository'; import {getJsonSchema} from '@loopback/repository-json-schema'; @model() class Category { @property() name: string; } @model() class Product { @property({required: true}) name: string; @property() type: Category; } const jsonSchema = getJsonSchema(Product); ``` `jsonSchema` from above would return: ```json { "title": "Product", "properties": { "name": { "type": "string" }, "type": { "$ref": "#/definitions/Category" } }, "definitions": { "Category": { "properties": { "name": { "type": "string" } } } }, "required": ["name"] } ``` If a custom type is specified for a decorated property in a model definition, then a reference [`$ref`](http://json-schema.org/latest/json-schema-core.html#rfc.section.8) field is created for it and a `definitions` sub-schema is created at the top-level of the schema. The `definitions` sub-schema is populated with the type definition by recursively calling `getJsonSchema` to build its properties. This allows for complex and nested custom type definition building. The example above illustrates this point by having the custom type `Category` as a property of our `Product` model definition. #### Supported JSON keywords {% include note.html content=" This feature is still a work in progress and is incomplete. " %} Following are the supported keywords that can be explicitly passed into the decorators to better tailor towards the JSON Schema being produced: | Keywords | Decorator | Type | Default | Description | | ----------- | ----------- | ------- | ------------ | ------------------------------------------------------- | | title | `@model` | string | _model name_ | Name of the model | | description | `@model` | string | | Description of the model | | array | `@property` | boolean | | Used to specify whether the property is an array or not | | required | `@property` | boolean | | Used to specify whether the property is required or not | ## Other ORMs You might decide to use an alternative ORM/ODM in your LoopBack application. LoopBack 4 no longer expects you to provide your data in its own custom Model format for routing purposes, which means you are free to alter your classes to suit these ORMs/ODMs. However, this also means that the provided schema decorators will serve no purpose for these ORMs/ODMs. Some of these frameworks may also provide decorators with conflicting names (e.g. another `@model` decorator), which might warrant avoiding the provided juggler decorators.