GRDB.swift: Crash when observing virtual table?

What did you do?

I have two tables, one that contains just the contents of a specific record type and another that is a FTS4 search index table of said the previously mentioned content.

Both tables have a corresponding type (struct conforming to TableRecord and FetchableRecord). The type that represents a row in the “regular” table defines a hasOne association with the search index type.

When performing a search in the app I create a QueryInterfaceRequest that among other things joins on the hasOne association which allows me to filter the request on matches in the index table.

It looks like this:

RecipePreview
            .joining(required: RecipePreview.index)
            .order(RecipePreview.Columns.name)
            .filter(filter.filter)
            .filter(tags: tags)
            .filter(query: query)

The query filter is just this extension on QueryInterfaceRequest:

    func filter(query: String?) -> QueryInterfaceRequest {
        guard let query = query?.trimmingCharacters(in: .whitespacesAndNewlines), !query.isEmpty else {
            return self
        }
        return self.filter(sql: "recipeSearchIndex.corpus MATCH ?", arguments: ["*\(query)*"])
    }

What did you expect to happen?

In most cases this works great and allows me to use a UISearchController to populate a collection view with the search results. However, I also observe this request because these items can be “favorited”, among other things, which changes the display of the item in the list.

What happened instead?

Whenever a search is performed while the search index is update I get a fatalError on line 201 of DatabaseRegion.swift stating:

precondition failure: event was not filtered out in observes(eventsOfKind:) by region.isModified(byEventsOfKind:)

Environment

GRDB flavor(s): GRDB GRDB version: 4.2.1 Installation method: Manual (git submodule) Xcode version: 11 Swift version: 5.1 Platform(s) running GRDB: iOS macOS version running Xcode: 10.15 beta… 8?

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I realise this probably has something to do with the virtual table. I guess my main questions is: is this a bug or intended behaviour? And can it be worked around somehow?

Thanks