Allow getFieldValue to read from foreign Reference objects. #5651
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The getFieldValue(fieldName) helper function was introduced in #5617 for reading fields from the current StoreObject during read functions. This commit adds a second parameter to getFieldValue, foreignRef, which is an optional Reference. When foreignRef is provided, getFieldValue will read the specified field from the StoreObject identified by the foreignRef, instead of reading from the current StoreObject. In either case, getFieldValue reads an existing value from the cache, without invoking any read functions, so you cannot use getFieldValue to set up expensive (and potentially cyclic) chains of read functions. With this new ability to read fields from arbitrary Reference objects, read functions can explore the entire reachable cache, without having to call cache.readQuery. The beauty of this system is that every field read operation requires a function call (getFieldValue), which allows the result caching system to know which fields were read from which entities, so future changes to those fields can properly invalidate any cached results that involved the original read function.
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The
getFieldValue(fieldName)helper function was introduced in #5617 for reading fields from the currentStoreObjectduringreadfunctions.This commit adds a second parameter to
getFieldValue,foreignRef, which is an optionalReference. WhenforeignRefis provided,getFieldValuewill read the specified field from theStoreObjectidentified by theforeignRef, instead of reading from the currentStoreObject.In either case,
getFieldValuereads an existing value from the cache, without invoking anyreadfunctions, so you cannot usegetFieldValueto set up expensive (and potentially cyclic) chains ofreadfunctions.With this new ability to read fields from arbitrary
Referenceobjects,readfunctions can explore the entire reachable cache without having to callcache.readQuery. The beauty of this system is that every field reading operation requires a function call (getFieldValue), which allows the result caching system to know which fields were read from which entities, so future changes to those fields can properly invalidate any cached results that involved the originalreadfunction.I recommend reading through the included tests to get a concrete sense for how everything works.