Unique property throws error when using mongo

**Waterline version**:0.13.6
**Node version**:10.12.0
**NPM version**:6.4.1
**Operating system**:Ubuntu 18.10

---

Related to - https://github.com/balderdashy/waterline/issues/1541, but no advice was given.

Currently trying to run the following code with sails-mongo. Have also tried with sails-disk and sails-memory and it is giving the same issue.

The following code results in the error: `the attribute `email` on the `user` model contains invalid properties. The property `unique` isn't a recognized property.`

```
export const userCollection = Waterline.Collection.extend({
identity: 'user',
datastore: 'default',
primaryKey: 'id',
dontUseObjectIds: true,

attributes: {
id: {
type: 'string',
required: true
},
firstName: {type: 'string'},
lastName: {type: 'string'},
email: {type: 'string', required: true, unique: true}
password: {type: 'string', required: true},

roles: {
collection: 'role'
}
}
}

```

```

const sailsMongoAdapter = require('sails-mongo');

export const datastoreConfig = {
adapters: {
'mongo': sailsMongoAdapter
},

datastores: {
default: {
adapter: 'mongo',
url: 'mongodb://mongo:27017/db',
schema:true
}
},
};

```

```

waterline.initialize(datastoreConfig, (err, ontology)=> {
if (err) {
console.error(err);
throw err;
}
}
```

Start with the Waterline.Collection.extend model definition and the waterline.initialize call shown in the report, then reproduce the validation error with the stated versions and configuration. Compare the behavior across sails-mongo, sails-disk, and sails-memory; done means the reported valid property is either accepted consistently or the issue is narrowed to a documented incompatibility.

Written by the indexing model from the issue text.