Document using "this" instead of "data" args in beforeValidate model hook

[aantipov]

Hi,
I use loopback version 2.10.2.
I have a model with a base "PersistedModel" and defined some properties on it.

{
  "name": "Hike",
  "base": "PersistedModel",
  "idInjection": true,
  "properties": {
    "title": {
      "type": "string",
      "required": true
    },
    "desc": {
      "type": "string"
    },
    "created": {
      "type": "date",
      "required": true
    }
  },
  "validations": [],
  "relations": {
    "author": {
      "type": "belongsTo",
      "model": "Account",
      "foreignKey": "authorId"
    }
  },
  "acls": [],
  "methods": []
}

Note that property created is required.

Then in model hook I try to set created value before validation:

Hike.beforeValidate = function (next, data) {
    data.created = new Date();
    next();
  };

After this I try to save a new object without created property & get validation error:

{
  "error": {
    "name": "ValidationError",
    "status": 422,
    "message": "The `Hike` instance is not valid. Details: `created` can't be blank (value: undefined).",
    "statusCode": 422,
    "details": {
      "context": "Hike",
      "codes": {
        "created": [
          "presence"
        ]
      },
      "messages": {
        "created": [
          "can't be blank"
        ]
      }
    },
    "stack": "ValidationError: The `Hike` instance is not valid. Details: `created` can't be blank (value: undefined)..."
  }
}

Here is a screenshot from api explorer with a post operation http://take.ms/iQdeV

Why was my model not actually changed before validation?

[raymondfeng]

@superkhau Can you try to reproduce it?

[raymondfeng]

@aantipov You need to update this instead:

Hike.beforeValidate = function (next, data) {
   // this is the model instance you are validating
    this.created = new Date();
    next();
  };

Please note data is original data object from the create().

[aantipov]

Thanks, @raymondfeng
But what is the purpose of the data object then?

[superkhau]

@aantipov You can use the data object is the model instance you're about to create. You can do whatever you need to do before the validation happens in your case (ie. change a property, etc)

[aantipov]

@superkhau But we have found out above that changing a property of a data object do not change the model data that are eventually saved - changing data object really matters nothing.
@raymondfeng advised to change this object instead.
Thus I'm confused about the purpose of data object.

[superkhau]

@aantipov I see what you mean now. That is a good question. @raymondfeng ?

[raymondfeng]

There is some legacy code we inherited from jugglingdb. We are cleaning up over time. In some case, we have to introduce breaking changes.

[superkhau]

@raymondfeng Maybe we should document this up front and say the recommended way is to use "this" amd ignore the "data" args due to legacy reasons?

/cc @crandmck

[crandmck]

OK, rather than open a new issue, I'm just going to reopen this as a doc issue. Will change title, label, etc., accordingly.

[aantipov]

Hi, @raymondfeng , @superkhau
I've just tried to change a modelInstance in Account.beforeCreate and it works fine:

Account.beforeCreate = function(next, modelInstance) {
    modelInstance.name = 'some-name';
    next();
};

So the questions emerged are:

• is this behavior a bug?
• should I use this.name = 'some-name' instead of modelInstance.name = 'some-name'
• if this hook initiates a call next(null, someObject) which data will be finally used in the method?