Add custom function support to predicates and sort descriptors#22
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Introduce a generic function-call expression so predicates and sort
descriptors can invoke named functions registered with a store, and add
a cross-backend registration API for those functions.
- Add FetchRequest.Predicate.Expression.function(FunctionExpression),
a generic {name, arguments} call expression.
- Add FetchRequest.SortDescriptor.term (SortTerm: .property/.function)
so results can be sorted by a function result, keeping a compatibility
init(property:ascending:) and .property accessor for existing callers.
- Add DatabaseFunction (name, argumentCount, deterministic, evaluate)
and a ModelStorage.register(function:) requirement, implemented by all
existing conformers. CoreData records functions for later in-memory
evaluation and maps .function to NSExpression(forFunction:).
Use NSManagedObjectContext.userInfo, the dictionary CoreData provides for associating custom data with a context, instead of objc associated objects.
CoreData can't run custom functions in a native fetch, so a fetch whose predicate or sort references a .function expression is split: a superset is fetched natively (function comparisons replaced with TRUE, function sorts and limit/offset dropped), then filtered, sorted, and paginated in memory using the registered functions. Adds a test registering a function on the CoreData backend and using it to filter and sort a collection of people by name.
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Summary
Adds a generic function-call mechanism to the abstract query model so predicates and sort descriptors can invoke named functions registered with a store.
Fixes #21.
Changes
Expression.function(FunctionExpression)— a new generic{ name: String, arguments: [Expression] }call expression (Expression.swift,FunctionExpression.swift). Any comparison against a function's result uses the existing operators, e.g.myFn(a, b) <= x.SortDescriptor.term: SortTerm— sort descriptors can now sort by either a plain.propertyor a.functionresult. A compatibilityinit(property:ascending:)and a.propertyaccessor keep all existing call sites source-compatible.DatabaseFunction— a value type carryingname,argumentCount,deterministic, and a backend-agnosticevaluate: ([AttributeValue?]) -> AttributeValue?closure.ModelStorage.register(function:)— a new protocol requirement, implemented by all existing conformers. A SQL-backed store can execute the function natively; CoreData records it for in-memory evaluation and maps.functiontoNSExpression(forFunction:).CoreData's actual in-memory evaluation of
.functionpredicates/sorts is deferred to a follow-up; this PR establishes the abstract API and wiring.Testing
DatabaseFunctionTestscoveringCodableround-trips of the new expression/sort cases, theevaluateclosure, and the compatibility initializer.