Exception[1] is thrown when function lookup() is used in SQL query. 

Mongo translator doesn't support selection of constants only(select 1, select 'hi' from smalla), which causes the exception.

[1]  3:59:20,997 ERROR [org.teiid.CONNECTOR] (Worker5_QueryProcessorQueue6) Connector worker process failed for atomic-request=Ef2Z7E0246by.1.5.3: com.mongodb.CommandFailureException: { "serverUsed" : "localhost/127.0.0.1:27017" , "errmsg" : "exception: $projection requires at least one output field" , "code" : 16403 , "ok" : 0.0}
	at com.mongodb.CommandResult.getException(CommandResult.java:71)

Server log is in the attachment.

Sample query:
SELECT BQT1.SmallA.IntKey FROM BQT1.SmallA, BQT1.SmallB WHERE BQT1.SmallA.IntKey= lookup('BQT1.SmallB', 'IntKey', 'StringKey', BQT1.SmallA.StringKey) ORDER BY IntKey

Generated mongo commands:

{ aggregate: "smalla", pipeline: [ { $project: { _m0: "$INTKEY", _m1: "$STRINGKEY" } } ] }

and 

{ aggregate: "smallb", pipeline: [ { $project: {} } ] }