mongo-why

Tells you why mongo fails to validate your document.

In a project of Appier, we use mongodb validator as a "schema". However, as the "schema" grows, it is a pain to see which part of a document fails validation. why.js is a library that can be loaded inside mongo shell and provides a function called why(), which shows a list of validator rules that the doc has violated.

Use case

Load why.js into mongo shell, then use why(<name of collection>, <the document you want to insert>)

$ mongo --shell path/to/why.js
MongoDB shell version: 3.2.4
type "help" for help
> why('users', {name: 123})
-------------------------
Unmatched validator rules
-------------------------
[
  {
    "name": {
      "$type": "string"
    }
  },
  {
    "password": {
      "$exists": true
    }
  }
]

Under the hood, it applies parts of the specified collection's validator to a temporary collection, and tries inserting the document into that temporary collection. Then it prints out the parts of the validator that cannot validate the document.

You may also specify a validator directly:

$ mongo --shell path/to/why.js
MongoDB shell version: 3.2.4
type "help" for help
> validator = {name: {$type: 'string'}, password: {$exists: true}}
> why(validator, {name: 123})
-------------------------
Unmatched validator rules
-------------------------
[
  {
    "name": {
      "$type": "string"
    }
  },
  {
    "password": {
      "$exists": true
    }
  }
]

Options

The third argument for why() is for options. The options hash and their default value are as follows:

why('CollectionName', documentToTest, {
  // when true, it returns unmached validator rules as arrays instead of printing
  // them out, so that you can customize the error output
  //
  quiet: false,
})

Development

There is only one file, why.js, a script that runs in mongo shell. It only has limited ES2016 support, this should be taken into account during development.

Unit Test

$ mongo test.js
MongoDB shell version: 3.2.4
connecting to: test
# If there is no assertion errors, the test passes.