Description:

This is just part of the broader $out project. The restrictions on the uniqueKey imposed by these code changes are like the description says:

In the "uniqueKey" spec:

• no duplicate keys
• no nested objects (use the dotted "nested.field" syntax instead)
• all fields must have value 1

At runtime:

• Each document must have every field of the uniqueKey. No fields of the uniqueKey may be missing, null, undefined or an array.

Engineering Ticket Description:

• Duplicate keys
• No nested objects
• All fields must have value '1'
• None of the values may be nullush