fix(repository): allow model classes with recursive type references

[raymondfeng]

Fixes #3671 (comment)

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• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
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• Affected example projects in examples/* were updated

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[jannyHou]

👍 LGTM

[jannyHou]

Some code change is probably not compatible with mongodb connector: https://travis-ci.com/strongloop/loopback-next/jobs/234512187

[raymondfeng]

Some code change is probably not compatible with mongodb connector: https://travis-ci.com/strongloop/loopback-next/jobs/234512187

Fixed.

[bajtos]

Can we leverage DataSource's ModelBuilder instance that's already keeping track of models that have been defined, instead of creating a new map?

[raymondfeng]

Probably not as ModelBuilder has no idea of LB4 entity class.

[bajtos]

ModelBuilder is keeping track of all models that are backing LB4 entity classes. Whenever we create a new Repository for an LB4 entity class, a new (Persisted)Model is registered with the ModelBuilder behind the datasource.

[raymondfeng]

@bajtos PTAL

[bajtos]

I am not very happy about the current implementation, because it introduces multiple places that are keeping track of visited models (visited, dataSource.modelBuilder.models).

Juggler already has means for handling circular references in models. I simplified your solution to leverage the concept of "unresolved models", now the fix consists of one new line added.

@raymondfeng PTAL.

If you agree with my proposed direction, then I am proposing the following next steps:

1. Fix juggler typings to include forceCreate parameter for dataSource.getModel(). See the source.
2. Publish a new juggler version.
3. In this patch: remove the cast to any, squash the commits. Please preserve my co-authorship via Co-authored-by trailer, see GitHub docs.

[raymondfeng]

@bajtos Please approve loopbackio/loopback-datasource-juggler#1778 first.