Expose explicit conflict when same ingredient+unit appears with different amounts

[dgee2]

Why

When a recipe request contained the same ingredient+unit combination more than once with different amounts, the API would crash with HTTP 500. NormalizeRecipeIngredients used Distinct() (structural equality on all three fields: ingredient name, unit, and amount), so exact duplicates were silently absorbed, but entries that differed only in amount both passed through. Both then entered UpsertRecipeIngredientsAsync, where each attempted to insert a row with the same composite primary key (RecipeId, IngredientId, UnitId), causing an unhandled DbUpdateException and a 500 response.

This is a business-significant conflict: a recipe listing the same ingredient and unit twice with contradictory amounts is logically invalid, and the user must resolve the ambiguity before the request can succeed.

What changed

RecipeService.NormalizeRecipeIngredients now performs conflict detection after the existing Distinct() deduplication pass. Any (IngredientName, UnitName) group that still has more than one entry (meaning the same ingredient+unit with different amounts) triggers a BusinessValidationException that names all conflicting pairs. BusinessValidationExceptionHandler converts this to HTTP 422, which both recipe endpoints already advertise in their OpenAPI contract.

The ordering matters: exact duplicates are still silently absorbed first (behaviour from #977), then the conflict check runs over what remains. No repository, schema, or OpenAPI contract changes are required.

Tests

• Unit tests (4 new): verify that CreateRecipeAsync and UpdateRecipeAsync throw BusinessValidationException and make no repository write calls when conflicting amounts are present; verify that all conflicting pairs are reported in the message; verify that mixed exact-duplicate + conflicting-amount input is still detected as a conflict.
• Integration tests (2 new): verify that POST /recipe and PUT /recipe/{id} return HTTP 422 with a problem detail when the same ingredient+unit appears with different amounts.

Fixes: #980

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

This PR prevents an HTTP 500 when a recipe request includes the same ingredient+unit more than once with different amounts by detecting that business conflict in RecipeService and returning a client-visible validation error (HTTP 422) instead.

Changes:

• Add conflict detection in RecipeService.NormalizeRecipeIngredients after exact-duplicate deduplication, throwing BusinessValidationException for ingredient+unit pairs that appear with multiple amounts.
• Add unit test coverage to ensure conflicts throw and repository writes are not invoked.
• Add integration tests to verify POST /api/recipe and PUT /api/recipe/{id} return HTTP 422 with problem details for conflicting ingredient amounts.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
backend/MenuApi/Services/RecipeService.cs	Adds ingredient+unit conflict detection after exact-duplicate removal and throws BusinessValidationException to prevent DB constraint crashes.
backend/MenuApi.Tests/Services/RecipeServiceTests.cs	Adds unit tests for conflicting ingredient amount detection and verifies no repository writes occur on conflict.
backend/MenuApi.Integration.Tests/RecipeWithIngredientsIntegrationTests.cs	Adds integration tests asserting HTTP 422 problem details for create/update requests with conflicting ingredient amounts.

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