Add __parent reference to embedded models

[bajtos]

This pull request supersedes #1787 by @mitsos1os. I cleaned up the commit history and also tweaked code formatting (white space) in few places.

Add a new hidden property __parent that's automatically set on all instances of embedded models.

For backwards compatibility, this feature is not enabled by default. You can turn it on by adding the following line to server/server.js file:

app.registry.modelBuilder.settings.parentRef = true;

Resolve #1787

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• Commit messages are following our guidelines

[bajtos]

Here are the test failures I am observing in the PostgreSQL, Cloudant and MongoDB connectors:

1) juggler-v4
     basic-querying
       find
         check __parent relationship in embedded models
           should fill the parent in embedded model:
   TypeError: user.should.have.property(...).which.has.property(...).which.is.instanceof(...).and.equals is not a function
    at Context.it (node_modules/loopback-datasource-juggler/test/basic-querying.test.js:968:33)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)

2) juggler-v4
     basic-querying
       find
         check __parent relationship in embedded models
           should assign the container model as parent in list property:
   TypeError: user.should.have.property(...).which.has.property(...).which.is.instanceof(...).and.equals is not a function
    at Context.it (node_modules/loopback-datasource-juggler/test/basic-querying.test.js:973:33)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)

3) juggler-v4
     basic-querying
       find
         check __parent relationship in embedded models
           should have the complete chain of parents available in embedded list element:
   TypeError: userFriend.should.have.property(...).which.equals is not a function
    at user.friends.forEach (node_modules/loopback-datasource-juggler/test/basic-querying.test.js:978:61)
    at Array.forEach (<anonymous>)
    at Context.it (node_modules/loopback-datasource-juggler/test/basic-querying.test.js:977:22)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)

[bajtos]

@slnode test please

[bajtos]

Trying to drop support for Node.js 8.x in the PostgreSQL connector to see if it makes any difference: loopbackio/loopback-connector-postgresql#431

[bajtos]

Hmm, by removing support for Node.js 8.x from the PostgreSQL connector I have effectively disabled dowstream testing :(

Let's wait for @rmg to update our downstream builders to use Node.js 12.

[bajtos]

@slnode test please

[bajtos]

Forgotten comment, will remove.

[bajtos]

Looks like 5dc158c fixed downstream tests, yay 🕺 I am going to clean up the commit history to run the final CI build and prepare this PR for landing.

[mitsos1os]

@bajtos kudos for finding the problem! Changes are good as far as I am concerned! Feel free to move on with the PR!