Unique indexes enforce uniqueness across indexed keys. Null, missing values and partial coverage need deliberate modeling.
Before you start
You should understand documents, collections and indexes. Sketch representative documents and the reads and writes they must support. MongoDB-specific examples assume a collection with the shown fields; deployment topology, permissions and existing indexes can affect the operational behavior being discussed.
The practical goal is to reason through this situation: Enforce uniqueness for a normalized username or a tenant-scoped identifier. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Define uniqueness scope
A tenant-local username differs from a globally unique username.
Step 2: Normalize consistently
Store the value according to the intended case and whitespace policy.
Step 3: Model optional coverage
Decide how missing and null values interact with the chosen index.
Worked scenario
Enforce uniqueness for a normalized username or a tenant-scoped identifier.
Two clients both check that a username is unused, then insert simultaneously. Database enforcement must settle that race. Optional usernames need deliberate index coverage; a partial unique index may fit the contract, but only if its filter correctly includes the values whose uniqueness matters.
Common mistake
An application-only existence check races with concurrent insertion.
Verify the behavior
Test concurrent duplicates, case variants, missing fields and null under the actual index definition.
Interview exercise
Handle optional unique fields.
Answer and reasoning
Define the index’s coverage and normalization, then test missing, null and duplicate values before deployment.
Continue learning
Compare the scenario with the MongoDB interview questions and test your understanding with the MongoDB MCQs. For terminology and implementation details, consult the reference material.