using MongoDB.Bson;
using MongoDB.Driver;

var client = new MongoClient();
var db = client.GetDatabase("test");
var coll1 = db.GetCollection<BsonDocument>("coll1");
var coll2 = db.GetCollection<BsonDocument>("coll2");

var query = coll1.AsQueryable().Union(coll2.AsQueryable());

var results = query.ToList();

Unhandled exception. MongoDB.Driver.Linq.ExpressionNotSupportedException: Expression not supported: test.coll1.Aggregate([]).Union(test.coll2.Aggregate([])).
   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)
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.Execute()
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.GetEnumerator()
   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/james/Dropbox/code/cases/csharp4627/Program.cs:line 11