fix(optimizer): Avoid merging prefetches when using aliases

[bellini666]

Merging querysets is usually fine, but when using an alias it might be that one queryset is filtering for something, and the other is filtering for something else, for the same field. That can lead to wrong results being returned.

From now on, if a field is specified more than once with an alias, the optimizer will skip it, possibly causing n+1 issues, but avoiding wrong results (which is worse).

Fix #695

Summary by Sourcery

Fix issue where using aliases with the same field could lead to incorrect results. Skip prefetching for fields specified multiple times with aliases to prevent this issue, which may cause n+1 problems.

Bug Fixes:

• Fixed a bug where using aliases with the same field in a query could lead to incorrect results.

Tests:

• Added a test case for queries with nested connections, filters, and aliases.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request modifies the query optimizer to avoid merging prefetches when using aliases. This prevents incorrect results when filtering on the same field with different aliases, but may lead to n+1 issues in some cases.

Sequence diagram for query optimization with aliases

sequenceDiagram
    participant Client
    participant Optimizer
    participant QuerySet

    Client->>Optimizer: Execute query with aliases
    Note over Optimizer: Check field selection count
    alt Field has multiple selections (aliases)
        Optimizer-->>QuerySet: Skip optimization for field
        Note over QuerySet: May cause N+1 queries
    else Field has single selection
        Optimizer->>QuerySet: Apply optimization
        Note over QuerySet: Optimized query execution
    end
    QuerySet-->>Client: Return results

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Flow diagram for field optimization decision

flowchart TD
    A[Start Field Processing] --> B{Field count > 1?}
    B -->|Yes| C[Skip Optimization]
    B -->|No| D[Apply Optimization]
    C --> E[Add parent relation fields]
    D --> E
    E --> F[Return Optimized Store]

    style C fill:#ffcccc
    style D fill:#ccffcc

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File-Level Changes

Change	Details	Files
Avoid merging prefetches when using aliases	The optimizer will now skip fields that are specified more than once with an alias. Added a test case to verify that the optimizer does not merge prefetches when using aliases. Added a test case to verify that the optimizer does not cause n+1 issues when skipping optimization for a relation.	tests/test_optimizer.py strawberry_django/optimizer.py
Removed redundant fields from test cases	Removed milestoneAgain field from test_query_forward_with_fragments test case. Removed otherIssues field from test_query_prefetch_with_fragments test case.	tests/test_optimizer.py

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#695	Resolve the issue of same prefetch_related query with different filters returning identical results when optimizer is enabled	✅
#695	Prevent incorrect query results when using field aliases with different filters	✅
#695	Ensure query optimization does not compromise query accuracy	✅

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[sourcery-ai]

Hey @bellini666 - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.12%. Comparing base (610e12b) to head (ba97980).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #698      +/-   ##
==========================================
- Coverage   89.05%   88.12%   -0.93%     
==========================================
  Files          42       42              
  Lines        3837     3850      +13     
==========================================
- Hits         3417     3393      -24     
- Misses        420      457      +37     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[NT-Timm]

Edit:
Nevermind. After thinking it over manually caching it would remove any problems with future library changes.