When asigning a value to a Schema property with a nested array type the schema appears to mutate the input incorrectly.

[hally9k]

Do you want to request a feature or report a bug?
Report a Bug.

What is the current behavior?
If a variable with the incorrect array nesting is assigned to a schema instance property it mutates the input instead of throwing a validation error.

It feels unhygienic to mutate the schema instance in this way but the resulting behaviour here is unexpected...

If the current behavior is a bug, please provide the steps to reproduce.
See the reproduction here.
https://codesandbox.io/s/sparkling-rain-dbv7v?fontsize=14&hidenavigation=1&theme=dark

The assignment on line 16:

position.coordinates = [1, 2];

Results in the output:

"coordinates": [
    [
      1
    ],
    [
      2
    ]
  ]

What is the expected behavior?
An error should be thrown when the incorrect schema object is assigned?

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
node@11.7.0
mongoose@5.8.10
mongodb@4.2

[jloveridge]

@hally9k mongoose has fairly complex typecasting logic. The current behavior of taking a single value and casting it into an array when the schema defines the property to be an array of values was explicitly requested, thus the current behavior that you are seeing. Any change to this behavior would be a backwards breaking change. I definitely understand you concern and frustration with the current behavior. However, changing how typecasting works should not be undertaken lightly. Better to ensure your code sets the appropriate values, casted however is appropriate, than to rely on typecasting behaviors inside of mongoose.

[vkarpov15]

We fixed it so that Mongoose will cast [1, 2] -> [[1, 2]] rather than [[1], [2]] going forward.

If you want that to throw a CastError, you can set the castNonArrays option to false:

const mongoose = require('mongoose');

mongoose.Schema.Types.Array.options.castNonArrays = false;

const positionSchema = mongoose.Schema({
  coordinates: {
    type: [[Number]],
    required: true
  }
});

const Position = mongoose.model("Position", positionSchema);

const position = new Position();

position.coordinates = [1, 2];

// CastError
console.log(position.validateSync());