Data Integrity Constraints
Enforce your important rules in the database, not just in the form.
What it adds
Database-level uniqueness, foreign key, check, and not-null constraints backing the business rules the application currently only validates in code.
What your agent is told to do
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What your agent is told to do
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Inventory the rules the app relies on — required fields, uniqueness, valid enum values, positive amounts, referential links — and note which exist only in application code.
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Before adding each constraint, query for rows that already violate it and decide explicitly whether to fix, delete, or exclude them. Every mature database has violating legacy rows.
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Add constraints in a way that does not hold a long exclusive lock: create indexes concurrently, add checks as NOT VALID and validate them in a second step.
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Keep application validation in place alongside the constraint so users get a field-level message, and catch the database error as a backstop for races and imports.
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Translate constraint violations into human sentences naming the field. A raw unique-index error string must never reach the user.
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Do NOT add a plain unique index to a table with soft deletes or multiple tenants without scoping it. A global unique email on a table with deleted rows will block a user from ever signing up again.
Edge cases it handles
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Edge cases it handles
7- Uniqueness across nullable columns behaves differently than people expect — NULL is not equal to NULL, so duplicates slip through.
- Soft-deleted rows must be excluded from uniqueness via a partial index, or deletion becomes permanent blocking.
- Multi-tenant uniqueness is almost always scoped to the tenant, not global — check every unique rule for a missing tenant column.
- Foreign keys need an explicit delete behaviour: cascade, nullify, or restrict. Defaulting silently is how orphans or accidental mass deletes happen.
- A race between two simultaneous requests will defeat a check-then-insert; the database constraint is what actually stops it, so the code must handle the raised error.
- Seed data, fixtures, and bulk importers frequently bypass model validation and will start failing the moment constraints land — fix them in the same change.
- Adding a NOT NULL column to a large table without a default can rewrite the table; stage it as nullable, backfill, then constrain.
Definition of done
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Definition of done
9- Every business rule identified as critical has a matching database constraint.
- Existing violating rows were found and resolved before each constraint was added.
- Migrations avoid long exclusive locks on large tables.
- Uniqueness is correctly scoped for tenants, soft deletes, and nullable columns.
- Foreign keys declare an explicit delete behaviour.
- Constraint violations surface as field-level user messages, never raw database errors.
- Concurrent duplicate requests produce one row, not two.
- The feature matches the existing design system.
- No existing functionality is broken.
Related features
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Offer a strong password in one click wherever someone sets or changes credentials.
What it does
A generate control on password fields that produces a strong value, shows it, and hands it to the user safely.
How it works
- 1 Add the control to every place a password is set: sign-up, password change, password reset completion, and any admin screen that provisions credentials for someone else.
- 2 Generate the value in the browser using the platform's cryptographic random source. The value must never be produced on the server or sent anywhere.
- 3 Reveal the generated password by default so the user can record it, and offer a regenerate control beside it. A hidden generated password that the user cannot see is a password they will immediately reset.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/password-generator
Idempotent Form Submission
Idempotent Form Submission
Make one submit mean one result, even when the network retries.
What it does
Idempotency keys on high-impact writes so a double-click, a retry, or a refresh cannot create the same record twice.
How it works
- 1 Find the writes where a duplicate is expensive — payments, orders, invitations, provisioning, anything that sends money or email.
- 2 Generate an idempotency key when the form is rendered, send it with the submission, and persist it server-side alongside the result of the first successful attempt.
- 3 When the same key arrives again, return the stored result instead of doing the work a second time. When the same key arrives with a different payload, reject it — that is a client bug, not a retry.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/idempotent-form-submission
Client Version Compatibility
Client Version Compatibility
Handle the tab that has been open since two deploys ago.
What it does
The server tells the client which build it expects, and the client responds — quietly for an optional update, insistently when the old code can no longer talk to the API.
How it works
- 1 Stamp every build with a version identifier and return it on API responses, either in a header or a small metadata endpoint. The client compares it against the version it was built as.
- 2 Separate two cases and treat them differently. A newer build being available is an optional update — offer it and let the user finish what they are doing. An API contract the running client cannot satisfy is a hard incompatibility and must block further writes.
- 3 For an optional update, show an unobtrusive, dismissible prompt and apply the new version at the next natural navigation. Do not interrupt.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/client-version-compatibility
How it works
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Copy the link
Grab the Markdown instruction URL for this feature.
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Give it to your AI
Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
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It inspects, then implements
Your agent reads your existing app first, then adds the feature to fit it.
Works with your stack
These instructions are written to adapt. They tell the agent to detect your framework, match your existing design system, and reuse what you already have — rather than assuming a particular stack.
Need it tighter than that? Customize the feature and tell it exactly what you're running.