# SE-0385: Custom Reflection Metadata **URL:** \ **Category:** Proposal Reviews\ **Created:** [January 24, 2023, 7:28pm UTC](https://forums.swift.org/t/se-0385-custom-reflection-metadata/62777 "2023-01-24T19:28:50Z")\ **Posts on this page:** 1\ **Showing post:** 19
**Author:** ![tgoyne](https://avatars.discourse-cdn.com/v4/letter/t/5e9695/32.png) [@tgoyne](https://forums.swift.org/u/tgoyne)\ **Post date:** [January 27, 2023, 3:24am UTC](https://forums.swift.org/t/se-0385-custom-reflection-metadata/62777/19 "2023-01-27T03:24:23Z")
The concrete things Realm would use this for are: 1. Automatic model class discovery. If the user does not supply a schema when opening a database, we currently call `objc_copyClassList()` to find all subclasses of `RealmSwift.Object` (and some other things) and generate a schema from them. `objc_copyClassList()` has several issues, but the big functional problem is that it eagerly loads all linked dylibs. This hurts app startup time, increases general app memory usage, and can make extensions overshoot their memory budget by itself. 2. Advanced schema customization. For example, a property `@Persisted var foo: Int` creates an integer column named "foo" in the underlying database table. We'd like to support something like `@Persisted(named: "bar") var foo: Int` or `@Named("bar") @Persisted var foo: Int` to let the user specify that the underlying column should instead be named "bar". Doing this via a property wrapper requires storing the string "bar" on every instance of the object and not just the one instantiated for schema discovery, which is an unacceptable increase in memory usage. I don't think we'd use this proposal for anything else by itself. The problem with using it to avoid instantiating an object, using Mirror to slurp the properties, and then using `ivar_getOffset` to work around the lack of keypaths on Mirror is that we would still need the `@Persisted` property wrapper. It does some wacky things for schema discovery that could go away, but it's also just the thing that turns property accesses into database reads. This means that we'd need property declarations to be something like `@Discoverable @Persisted var foo: Int`, i.e. we'd be making our API worse and requiring users to write more boilerplate just to make our implementation simpler. It's possible that declaration macros fix this. We could perhaps have a single macro attribute that applies both required attributes to the property (or fully expands into getters and setters and skips the property wrapper entirely). * * * My opinion on the proposal is still roughly what it was for the pitch: I'm not wild about this specific design, but it is something we'd use if it existed and it'd solve both functional and API design issues we're currently facing. --- _[View the full topic](https://forums.swift.org/t/se-0385-custom-reflection-metadata/62777)._