fix(query): cast case and then expressions in $expr $switch branches

[kwy404]

Summary

cast$expr() passes each $switch branch object straight to _castExpression():

val.$switch.branches = val.$switch.branches.map(v => _castExpression(v, schema, strictQuery));

A branch is { case: <expression>, then: <expression> }. _castExpression() only acts on operator keys like $eq or $multiply, so on a branch object it finds nothing to do and returns it unchanged. The case and then expressions are never cast, while the if, then and else of $cond are.

This PR casts branch.case and branch.then the same way $cond casts if and then. Missing keys are skipped, so a malformed branch still goes to the server as is.

Examples

const Person = mongoose.model('Person', new Schema({ name: String, age: Number }));
await Person.create([{ name: 'a', age: 30 }, { name: 'b', age: 40 }]);

// before: matches nothing, because the '30' in case stays a string. After: matches 'a'
await Person.find({ $expr: { $switch: { branches: [{ case: { $eq: ['$age', '30'] }, then: true }], default: false } } });

// matches 'a' both before and after, because $cond.if is already cast
await Person.find({ $expr: { $cond: { if: { $eq: ['$age', '30'] }, then: true, else: false } } });

Added casts case and then expressions in $switch branches to test/helpers/query.cast$expr.test.js. It fails on master (the '18' in case and the '2' in then stay strings) and passes with this change. The existing gh-14751 $switch test and the other $expr tests still pass, and eslint is clean.

[Copilot]

Copilot review overview

🟡 Changes recommended

The new loop unintentionally stops removing undefined fields from branch objects.

Review effort: Balanced
Findings: 1

Open (1)

• Preserve omitUndefined cleanup for cast branch fields · New

What changed in this PR

Fixes casting within $expr.$switch branch expressions.

Changes:

• Casts each branch’s case and then expressions.
• Adds regression coverage for comparison and arithmetic casting.

File	Description
lib/​helpers/​query/​cast$expr.js	Recursively casts $switch branch expressions.
test/​helpers/​query.cast$expr.test.js	Tests casting within case and then.

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[kwy404]

Good catch, thanks. I confirmed that master drops then: undefined from the branch through the final omitUndefined() pass, while this loop kept it. I added omitUndefined(branch) after casting case and then, so branches get the same cleanup as before, and extended the test with a branch that has then: undefined.