Summary

We used LINQ to "Union" two collections and "Join" in a third one. When we tried that the query throws an error:

   System.InvalidOperationException : More than one possible serializer found for <>f__AnonymousType0`2[System.String,System.Boolean] in x.

This is true for versions 2.27.0, 2.26.0, 2.25.0 of the MongoDB.Driver nuget package.

How to Reproduce

write followin statement and try to execute it.

            var queryable = _firstCollection
                .AsQueryable()
                .Where(c => c.Name.StartsWith("second"))
                .Union(_secondCollection.AsQueryable()
                    .Where(c => c.Name.StartsWith("another")))
                .Where(x => x.Id == 2);

First of all, it's not possible to chain an Where after the Union because it does not return an "IMongoQuerable<T>". This is easy solved with:

        public static IMongoQueryable<TSource> Union<TSource>(this IMongoQueryable<TSource> source1, IEnumerable<TSource> source2)
        {
            Ensure.IsNotNull(source1, nameof(source1));
            Ensure.IsNotNull(source2, nameof(source2));
            return (IMongoQueryable<TSource>)Queryable.Union(source1, source2);
        }

But after that it throws the Exception from above.

I figured out an workaround how to fix it, but I am not able to run the whole set of unit tests that are available in the project, because it seems like it needs an local running mongodb to perform them.

So I don't know if this is a proper fix or there maybe an better solution for this issue.

I will create an PR with what I have and hopefully you can give some feedback on that.