context.Set<Order>().Select(e => e.Foo.FirstOrDefault()).ToList();

Unhandled exception. System.InvalidOperationException: MongoDB.Bson.Serialization.Serializers.StringSerializer must implement IBsonArraySerializer to be used with LINQ.
   at MongoDB.Driver.Linq.Linq3Implementation.Misc.ArraySerializerHelper.GetItemSerializer(IBsonSerializer serializer)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.MethodTranslators.FirstOrLastMethodToAggregationExpressionTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.MethodCallExpressionToAggregationExpressionTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ExpressionToAggregationExpressionTranslator.Translate(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(TranslationContext context, LambdaExpression lambdaExpression, Symbol parameterSymbol)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(TranslationContext context, LambdaExpression lambdaExpression, IBsonSerializer parameterSerializer, Boolean asRoot)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.SelectMethodToPipelineTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.ExpressionToPipelineTranslator.Translate(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators.ExpressionToExecutableQueryTranslator.Translate[TDocument,TOutput](MongoQueryProvider`1 provider, Expression expression, ExpressionTranslationOptions translationOptions)
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.Execute()
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.GetEnumerator()
   at MongoDB.EntityFrameworkCore.Query.QueryingEnumerable`2.Enumerator.MoveNextHelper()
   at MongoDB.EntityFrameworkCore.Query.QueryingEnumerable`2.Enumerator.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Program.<Main>$(String[] args) in /Users/arthur.vickers/code/OnYourOwnNow/OnYourOwnNow/Program.cs:line 11

using var context = new SomeDbContext();
context.Database.EnsureDeleted();

context.AddRange(new Order { Id = 1, Foo = "ABC" }, new Order { Id = 2, Foo = "DEF" });
context.SaveChanges();

var value = context.Set<Order>().Select(e => e.Foo.FirstOrDefault()).ToList();
Console.WriteLine(value.Count);

public class SomeDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseMongoDB("mongodb://localhost:27017", "SomeDb3")
            .LogTo(Console.WriteLine, LogLevel.Information);

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>();
    }
}

public class Order
{
    public int Id { get; set; }
    public string Foo { get; set; }
}