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Individual models are easy to understand and work with. But in reality, models are often connected or related. When you build a real-world application with multiple models, you’ll typically need to define relations between models. For example:

  • A customer has many orders and each order is owned by a customer.
  • A user can be assigned to one or more roles and a role can have zero or more users.
  • A physician takes care of many patients through appointments. A patient can see many physicians too.

With connected models, LoopBack exposes as a set of APIs to interact with each of the model instances and query and filter the information based on the client’s needs.

Model relation in LoopBack 3 is one of its powerful features which helps users define real-world mappings between their models, access sensible CRUD APIs for each of the models, and add querying and filtering capabilities for the relation APIs after scaffolding their LoopBack applications. In LoopBack 4, with the introduction of repositories, we aim to simplify the approach to relations by creating constrained repositories. This means that certain constraints need to be honoured by the target model repository based on the relation definition, and thus we produce a constrained version of it as a navigational property on the source repository. Additionally, we also introduce the concept of the inclusion resolver in relations, which helps to query data over different relations. LoopBack 4 creates a different inclusion resolver for each relation type.

Here are the currently supported relations:

The articles on each type of relation above will show you how to leverage the new relation engine to define and configure relations in your LoopBack application.

To generate a HasMany, HasOne, BelongsTo, or hasManyThrough relation through the CLI, see Relation generator.

Weak vs Strong Relations

LoopBack 4 implements weak relations with @belongsTo(), @hasMany(), @hasOne, etc. This means the constraints are enforced by LoopBack 4 itself, not the underlying database engine. This is useful for integrating cross-database relations, thereby allowing LoopBack 4 applications to partially take the role of a data lake.

However, this means that invalid data could be keyed in outside of the LoopBack 4 application. To resolve this issue, some LoopBack 4 connectors (such as PostgreSQL and MySQL) allow defining a foreign key constraint through the @model decorator. Please consult the respective connector documentation to check for compatibility.

Issue #2331 tracks native support for foreign key constraints in relation decorators (such as @belongsTo).


Filtering by parent model

Where filters such as those used by model queries (create(), find(), replaceById(), and so on) cannot be used to filter a model by the value of its parent model. See its GitHub issue.

Splitting numbers of queries

It doesn’t split numbers of queries. Related GH issue: Support inq splitting.

Handling of MongoDB ObjectID type

It might not work well with ObjectID of MongoDB. Related GH issue: Spike: robust handling of ObjectID type for MongoDB.