Concept questionMedium~7 minData Integrity & Transactions / 12 of 14

Interview question

How would you handle two requests trying to create the same unique value through EF Core?

Handle unique constraint races in EF Core APIs

Explains why application-level duplicate checks are not enough and how EF Core APIs map database constraints to friendly errors.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

I would keep the database unique constraint as the final authority. The API can check first to return a nicer message, but two requests can pass that check concurrently. When SaveChanges hits the unique constraint, EF Core raises a database update exception. I would catch the specific constraint violation, map it to a clear conflict or validation response, and avoid returning a generic 500.

Full answer below

Practice focus

Unique constraints, race conditions, DbUpdateException, tenant-scoped uniqueness, validation, and API error mapping.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Duplicate checks are a classic source of race conditions. EF Core code often looks correct until two requests hit the same path at the same time.
I would keep the database unique constraint as the final authority. The API can check first to return a nicer message, but two requests can pass that check concurrently. When SaveChanges hits the unique constraint, EF Core raises a database update exception. I would catch the specific constraint violation, map it to a clear conflict or validation response, and avoid returning a generic 500.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would model the exact uniqueness rule in the constraint. For example, a slug may be unique per tenant, not globally, and an email may need normalization before comparison. If soft deletes exist, the unique index may need a filtered condition or the product may forbid reuse. The application validation and database constraint need to describe the same rule, otherwise the API will behave inconsistently under load.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I would track constraint violations because spikes can indicate abuse, retry storms, or a frontend bug. During migrations, I would clean existing duplicates before adding the constraint. I would also keep error mapping narrow, based on known constraint names or database codes, so unrelated data errors do not become misleading user messages. Correctness belongs in the database, while clarity belongs in the API.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Relying only on a pre-insert query to check uniqueness.
Returning a 500 for a known duplicate-key error.
Forgetting tenant scope or normalization.
Mapping every database update exception as duplicate data.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot explain the race between check and insert.
Has no database constraint in the design.
Does not know how the API reports the conflict.
Ignores soft-delete and tenant-scope rules.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

How would you map a unique constraint error safely?
What makes a slug unique per tenant?
How would soft delete change the index?

Continue the section

Next in Data Integrity & Transactions: Tenant Filters

Step 13 of 14

Add guardrails around EF Core tenant query filters

Opening drill

Concept questionMedium~7 minData Integrity & Transactions / 12 of 14

Interview question

How would you handle two requests trying to create the same unique value through EF Core?

Handle unique constraint races in EF Core APIs

Explains why application-level duplicate checks are not enough and how EF Core APIs map database constraints to friendly errors.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Full answer below

Practice focus

Unique constraints, race conditions, DbUpdateException, tenant-scoped uniqueness, validation, and API error mapping.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Duplicate checks are a classic source of race conditions. EF Core code often looks correct until two requests hit the same path at the same time.
I would keep the database unique constraint as the final authority. The API can check first to return a nicer message, but two requests can pass that check concurrently. When SaveChanges hits the unique constraint, EF Core raises a database update exception. I would catch the specific constraint violation, map it to a clear conflict or validation response, and avoid returning a generic 500.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would model the exact uniqueness rule in the constraint. For example, a slug may be unique per tenant, not globally, and an email may need normalization before comparison. If soft deletes exist, the unique index may need a filtered condition or the product may forbid reuse. The application validation and database constraint need to describe the same rule, otherwise the API will behave inconsistently under load.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I would track constraint violations because spikes can indicate abuse, retry storms, or a frontend bug. During migrations, I would clean existing duplicates before adding the constraint. I would also keep error mapping narrow, based on known constraint names or database codes, so unrelated data errors do not become misleading user messages. Correctness belongs in the database, while clarity belongs in the API.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Relying only on a pre-insert query to check uniqueness.
Returning a 500 for a known duplicate-key error.
Forgetting tenant scope or normalization.
Mapping every database update exception as duplicate data.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot explain the race between check and insert.
Has no database constraint in the design.
Does not know how the API reports the conflict.
Ignores soft-delete and tenant-scope rules.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

How would you map a unique constraint error safely?
What makes a slug unique per tenant?
How would soft delete change the index?

Continue the section

Next in Data Integrity & Transactions: Tenant Filters

Step 13 of 14

Add guardrails around EF Core tenant query filters

How would you handle two requests trying to create the same unique value through EF Core?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue the section

How would you handle two requests trying to create the same unique value through EF Core?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue the section