INTPYTHON-750: Support converting $expr with $getField paths

[Jibola]

Summary

An addendum to INTPYTHON-739 This PR implements support for $getField expressions in the expression converter system, allowing MongoDB aggregation expressions using $getField to be optimized into simpler query conditions. This is especially useful when optimizing queries against embedded model fields.

The format of $getField calls need to meet these conditions:

{ 
    $getField: {
        input: "$path" | { $getField : {...} },  # A string starting with "$" 
        field: "path" # A string with no "$" or "." 
   }
}

This supports nested $getField operations.

Changes in this PR

• Adds helper methods to detect and convert $getField expressions to dot notation paths
• Updates binary and $in operators to handle $getField expressions alongside simple field references
• Adds comprehensive test coverage for $getField conversion scenarios

Test Plan

• Manual testing through python manage.py shell See an example query below.
• Added test cases.

Screenshots (grabbed text)

# Before
>>> NullableJSONModel.objects.filter(value__a="b", value__j=None)
{
                "$match": {
                    "$and": [
                        {
                            "$expr": {
                                "$eq": [
                                    {
                                        "$getField": {
                                            "input": "$value",
                                            "field": "a"
                                        }
                                    },
                                    "b"
                                ]
                            }
                        },
                        {
                            "$expr": {
                                "$eq": [
                                    {
                                        "$getField": {
                                            "input": "$value",
                                            "field": "j"
                                        }
                                    },
                                    null
                                ]
                            }
                        }
                    ]
                }
            }

# After 
>>> NullableJSONModel.objects.filter(value__a="b", value__j=None)
{
                "$match": {
                    "$and": [
                        {
                            "value.a": "b"
                        },
                        {
                            "$and": [
                                {
                                    "value.j": {
                                        "$exists": true
                                    }
                                },
                                {
                                    "value.j": null
                                }
                            ]
                        }
                    ]
                }
            }

Callouts

• This may be the last addition to the optimizer done as it extends the functionality of converting simple lookups on embedded model field paths.
• I decided not to convert every $getField call unless they also feed into a $match conversion because silently mutating $getField should best be dealt in the larger refactor.
• NULL checks have to explicitly check for both field existence and equality. As such, anytime a NoneType is used a lookup, the optimization needs to wrap two statements in an $and query.

[NoahStapp]

Can we optimize this down to remove the $getField when the field names don't contain $ or .? Something like:

{  
  "$match": {  
    "$expr": {  
      "$gt": ["$price.value", "$discounted_price.value"]  
    }  
  }  
}  

[Jibola]

We could. I'm trying to think if there's any case where this would be an issue if it mistakenly resolved it.

[Jibola]

This would have to be a two-phase conversion. I think it may even be best to still change it in our actual query code.

[NoahStapp]

I'm assuming here that this change ends up being needed. Holding off until we either get customer complaints or the refactor can't solve this makes more sense to me for now.

[Jibola]

Yeah, I tried to make it work (and I think I managed to), but the silent mutation of $getField to $field is worrying to me. Even if I manage to make things pass tests, it just feels out of scope of the optimizers goal, which is to explicitly change things from $expr to $match.

[Copilot]

Pull Request Overview

This PR implements support for $getField expressions in the expression converter system, allowing MongoDB aggregation expressions using $getField to be optimized into simpler query conditions.

• Adds helper methods to detect and convert $getField expressions to dot notation paths
• Updates binary and $in operators to handle $getField expressions alongside simple field references
• Adds comprehensive test coverage for $getField conversion scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
django_mongodb_backend/query_conversion/expression_converters.py	Core implementation of $getField support with path conversion methods and updated operator handling
tests/expression_converter_/test_op_expressions.py	Test cases for $getField conversion across all binary operators and $in operator
tests/expression_converter_/test_match_conversion.py	Integration tests for $getField optimization in match expressions

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[Copilot]

Use explicit tuple syntax (list, tuple, set) instead of union operator | for better compatibility with older Python versions.

Suggested change

	isinstance(values, list | tuple | set)
	isinstance(values, (list, tuple, set))

[Jibola]

closing in lieu of #396