Swift Core Data Query Generation Sqlite error code 769 ( sqlite3_snapshot_open failed ) - swift

Question

I'm using core data query generation,for each read on view context data i do the following
pin context
read (fetch request)
unpin context
Everything is working correctly as expected, but sometimes I get the following error:
error: SQLCore dispatchRequest: exception handling request: , sqlite3_snapshot_open failed with userInfo of {
NSSQLiteErrorDomain = 769;
}
CoreData: error: SQLCore dispatchRequest: exception handling request: , sqlite3_snapshot_open failed with userInfo of {
NSSQLiteErrorDomain = 769;
}
Below the code to pin/unpin the context:
// MARK: Query generation
func pinContext() {
do {
context.reset()
try context.setQueryGenerationFrom(NSQueryGenerationToken.current)
} catch {
fatalError("Context pinning:\(error)")
}
}
func unpinContext() {
do {
try context.setQueryGenerationFrom(nil)
} catch {
fatalError("Context unpinning:\(error)")
}
}
sqlite.org web site for error code (769) SQLITE_ERROR_SNAPSHOT states:
The SQLITE_ERROR_SNAPSHOT result code might be returned when attempting to start a read transaction on an historical version of the database by using the sqlite3_snapshot_open() interface. If the historical snapshot is no longer available, then the read transaction will fail with the SQLITE_ERROR_SNAPSHOT. This error code is only possible if SQLite is compiled with -DSQLITE_ENABLE_SNAPSHOT.
Now, how may I handle or prevent this error in Core data?

Answer 1

bgCTX = container.newBackgroundContext()
bgCTX.mergePolicy = NSMergePolicy.mergeByPropertyObjectTrump
bgCTX.automaticallyMergesChangesFromParent = true
try bgCTX.setQueryGenerationFrom(NSQueryGenerationToken.current)

Answer 2

Do you have automaticallyMergesChangesFromParent set to true? If so, that seems to cause issues. According to a comment in NSManagedObjectContext:
/* Whether the context automatically merges changes saved to its coordinator or
parent context. Setting this property to YES when the context is pinned to a non-
current query generation is not supported.
*/
#available(iOS 10.0, *)
open var automaticallyMergesChangesFromParent: Bool
If your context is pinned to the "current" query generation token, then it shouldn't be an issue. However, that query generation token could quickly become stale (non-current) if you are saving data to the store on other contexts. As soon as it tries to merge changes from the parent, it gets into an unknown state, which causes this error/crash.
From what I can tell, setting automaticallyMergesChangesFromParent to false when pinning a context is the safest route. Then you can set it to true again when unpinning.
I wish Apple would update their documentation with this, but instead we have to find out about it in a comment in their code.