What's the benefit of `BelongsTo` relation?

Question

What's the benefit of defining BelongsTo relation?

When I defined HasMany relation I could execute all my queries without missing anyone (I prefer to give me an example in case you find it's important to use BelongsTo relation).

Answer 1

The benefit is being able to flip the "base" and "child" in of the relation. LoopBack 4 relation filters are akin to an SQL LEFT JOIN, meaning that the filter must be scoped from the base model.

From the "Filtering by parent model" docs:

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.

For example, we may have many Orders made by a Customer:

@hasMany(() => Order)
orders?: Order[];

This relation enables querying for "orders made by a customer" or "orders made by a list of filtered customers", but not the other way around; "the customer that made that order" or "the customers that made a list of filtered orders".

A Belongs To relation solves this by creating a key on the Order that references a Customer:

@belongsTo(() => Customer)
customerId: number;

This means that we can now query "which customer made that order" or "which customers made the filtered list of orders".

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Another important factor is that a Has Many relation can't be made into a Strong Relation as ANSI SQL does not have a method of representing such relations. From the docs:

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.

Emphasis in italics is mine on why these restrictions apply.