How to set uniqueness for a field in MongoDB

MongoDB is a schemaless database.  As such, the possibilities for constraints are less than with a relational database.  Nevertheless, with the MongoDB validator it is possible to do validations.  They are based on the $jsonschema operator which itself is based on (actually is a subset/superset of) the JSON Schema draft-à4 specification.  

The JSON Schema specification allows to set uniqueness in arrays only, and in Hackolade, we support this via the corresponding property:

But this uniqueness feature is not available for scalar fields in a document.

However, you can set in MongoDB a unique index  on a field or a set of fields.  This is done in Hackolade, at the collection level, in the Indexes tab whe you can create your index, which will generate the corresponding script in forward-engineering.