score:2

Accepted answer

Profiler becomes your friend when you're not sure about driver syntax. To enable logging for all queries you need to run on your database below statement:

db.setProfilingLevel(2)

Then to check last query executed on the database you need to run:

db.system.profile.find().limit(1).sort( { ts : -1 } ).pretty()

So for the first code snippet you will get:

"pipeline" : [ { "$sort" : { "t" : 1 }  },
               { "$limit" : 1 }
]

"pipeline" : [ { "$sort" : { "t" : -1 }  },
               { "$limit" : 1 }
]

For the second pair it prints 

"pipeline" : [ { "$sort" : { "t" : 1 }  },
               { "$limit" : 1 }
]

and throws NotSupportedException for LastOrDefault on my machine, if it works on your MongoDB driver's version you can check generated MongoDB statement using profiler

For the last one when you hover on c in your Visual Studio it prints

{aggregate([{ "$sort" : { "t" : 1 } }])}

but since it's of type IOrderedQueryable<T> it is only not materialized query so it will get executed on the database when you run FirstOrDefault generating the same aggregation body as previous statements. I'm getting NotSupportedException here as well. Here you can find a list of supported LINQ operators and both Last and LastOrDefault are not implemented so you need to sort descending.


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