fix(optimizer): Prevent issuing duplicated queries for certain uses of first() and get()

[diesieben07]

For "many-type" fields (i.e. ManyToManyField or a reverse FK) when the corresponding GraphQL field has a singular result, StrawberryDjangoField will use QuerySet#get and QuerySet#first to produce the result. This prevents the QuerySet from using any prefetched data, because those methods do an implicit slice.
This PR adds a check to not call get/first in case the QuerySet is prefetched and instead mimic their behavior to prevent duplicated queries.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Documentation

Issues Fixed or Closed by This PR

Fixes #662

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

Summary by Sourcery

Fix duplicated queries in StrawberryDjangoField by avoiding get() and first() on prefetched QuerySets and add tests to verify the fix.

Bug Fixes:

• Prevent duplicated queries by avoiding the use of get() and first() on prefetched QuerySets in StrawberryDjangoField.

Tests:

• Add tests to verify the behavior of prefetched QuerySets with singular GraphQL fields, ensuring no duplicated queries occur.

[sourcery-ai]

Reviewer's Guide by Sourcery

This PR optimizes the handling of prefetched QuerySets in StrawberryDjangoField by preventing duplicate queries when using first() and get() methods. The implementation checks if a QuerySet is prefetched and, if so, mimics the behavior of these methods using the prefetched data instead of executing additional database queries.

Sequence diagram for optimized QuerySet handling

sequenceDiagram
    participant User
    participant StrawberryDjangoField
    participant QuerySet
    participant Database

    User->>StrawberryDjangoField: Request data
    StrawberryDjangoField->>QuerySet: Check if prefetched
    alt Prefetched
        QuerySet->>StrawberryDjangoField: Return prefetched data
        StrawberryDjangoField->>User: Return data
    else Not prefetched
        QuerySet->>Database: Execute query
        Database->>QuerySet: Return data
        QuerySet->>StrawberryDjangoField: Return data
        StrawberryDjangoField->>User: Return data
    end

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File-Level Changes

Change	Details	Files
Added optimization for prefetched QuerySets to prevent duplicate queries	Added check for prefetched QuerySets using is_optimized_by_prefetching() Implemented custom handling of first() using next(iter(qs), None) Implemented custom get() behavior with proper error handling for zero or multiple results	strawberry_django/fields/field.py
Added comprehensive test coverage for the new optimization	Added test for optional single field with multiple milestones Added test for required single field with existing issue Added test for required single field with missing issue Added test for required single field with multiple issues	tests/test_optimizer.py
Extended milestone schema with new field definitions	Added first_issue optional field Added first_issue_required field	tests/projects/schema.py

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#662	Fix the issue where prefetched objects are bypassed when using QuerySet#first/get in one-to-many relations that return a single type	✅

Possibly linked issues

• Optimizer results are not used in case of one-to-many relations where just one object is used #662: The PR fixes the optimizer issue described in the issue by preventing duplicated queries.

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[sourcery-ai]

Hey @diesieben07 - 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: 1 issue found
• 🟢 Documentation: all looks good

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[diesieben07]

@bellini666 I'm not sure why the type check is failing. What is "Error: Fields without default values cannot appear after fields with default values (reportGeneralTypeIssues)" referring to? Any guidance would be appreciated if you can.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.08%. Comparing base (1b49393) to head (913c5f6).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #675      +/-   ##
==========================================
+ Coverage   88.99%   89.08%   +0.09%     
==========================================
  Files          41       41              
  Lines        3752     3767      +15     
==========================================
+ Hits         3339     3356      +17     
+ Misses        413      411       -2     

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

[bellini666]

ty! :)

I've made a small adjustment to use qs._result_cache instead of len(qs). Although it should be ok, I think it's better to be safe than sorry 😅

[diesieben07]

Thank you! :)