Loopback-next: After adding a new field, how to get the default value into existing documents?

[joeytwiddle]

I have added a new field to my model:

  @property({
    type: 'boolean',
    default: true,
  })
  isFoo: boolean;

Now, when I fetch an old existing document from the collection, I want to see it has the property isFoo: true. How can we achieve that?

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Plan A: I fetch pre-existing instances of the model from MongoDB, but this new property is not present on the objects.

I was hoping to see isFoo: true on the objects I had fetched.

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Plan B: I fetched a document, and saved it back:

const document = await myModelRepository.findOne({});
await myModelRepository.update(document!);

I was hoping doing this would set the default, storing isFoo: true in the DB.

But I actually saw isFoo: null stored in the DB. 😱 That seems like a violation of contract to me! (Given that Loopback's validation would reject such a document if I sent it to the API.)

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Plan C: I tried to manually update the documents in the DB, using the defaults from new Model() but that object has no fields set at all, so I cannot see what the default value is supposed to be.

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Plan D: This is what I have needed to do in practice (and also what derdeka mentions below). I run a migration script that checks for any documents with that field unset, and updates the value to true.

(This is basically Plan C except in Plan C I found I was unable to get the default value out of the model, so I have to hard-code the true in my migration script.)

This has a few disadvantages, not least of which needing to repeat the value true in a second place in the codebase. (Violates SSOT)

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This was easy with Mongoose: Whenever documents were fetched, or written, any missing fields were populated with the default values.[1]

But as far as I can tell, Loopback 4 is only populating default values when I create a new document.

Is that the expected behaviour?

I built a fresh example project to ensure it wasn't just a problem with my project.

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What am I asking for?

• Discuss whether populating default values should be a feature when _reading_ and _writing_ entities, instead of only when _creating_. (This is what mongoose does, and it I have found it quite useful.)

• Alternatively, please recommend how to achieve the population of new fields when needed (e.g. as a migration step).

• Document the default property on the models page, when it is applied, and how to migrate old documents.

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[1] Mongoose will skip population of defaults with sparse: true or validate: false options, useful when dealing with large numbers of records.

[derdeka]

@joeytwiddle I have no idea how it works with mongodb, but in mysql and postgres I use database level defaults which are added when the migration process runs. After that they automatically return the default value.

[achrinza]

This could potentially be a use-case for #487

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