add support for dateToString expression

[parvanov]

There's no way currently to use $dateToString aggregation operator.

Projection.expression wraps all arguments as a list while dateToString expects a single object as argument.

I have tried all 3 examples bellow and all of them fail with the same exception:

DBObject format = new BasicDBObject("format", "%Y-%m-%d");
format.put("date", "$t");

Object format = new Object(){
public String format = "%Y-%m-%d";
public String date = "$t";
};

Object format = "{"format": "%Y-%m-%d", "date": "$t"}";

...
.project(projection("t", expression("$dateToString", format)))

Results in:

com.mongodb.MongoCommandException: Command failed with error 18629: '$dateToString only supports an object as its argument' on server localhost:27018. The full response is { "ok" : 0.0, "errmsg" : "$dateToString only supports an object as its argument", "code" : 18629 }
at com.mongodb.connection.ProtocolHelper.getCommandFailureException(ProtocolHelper.java:86)
at com.mongodb.connection.CommandProtocol.execute(CommandProtocol.java:119)
at com.mongodb.connection.DefaultServer$DefaultServerProtocolExecutor.execute(DefaultServer.java:159)
at com.mongodb.connection.DefaultServerConnection.executeProtocol(DefaultServerConnection.java:286)
at com.mongodb.connection.DefaultServerConnection.command(DefaultServerConnection.java:173)
at com.mongodb.operation.CommandOperationHelper.executeWrappedCommandProtocol(CommandOperationHelper.java:215)
at com.mongodb.operation.CommandOperationHelper.executeWrappedCommandProtocol(CommandOperationHelper.java:206)
at com.mongodb.operation.CommandOperationHelper.executeWrappedCommandProtocol(CommandOperationHelper.java:112)
at com.mongodb.operation.AggregateOperation$1.call(AggregateOperation.java:227)
at com.mongodb.operation.AggregateOperation$1.call(AggregateOperation.java:223)
at com.mongodb.operation.OperationHelper.withConnectionSource(OperationHelper.java:239)
at com.mongodb.operation.OperationHelper.withConnection(OperationHelper.java:212)
at com.mongodb.operation.AggregateOperation.execute(AggregateOperation.java:223)
at com.mongodb.operation.AggregateOperation.execute(AggregateOperation.java:65)
at com.mongodb.Mongo.execute(Mongo.java:773)
at com.mongodb.Mongo$2.execute(Mongo.java:760)
at com.mongodb.DBCollection.aggregate(DBCollection.java:1377)
at com.mongodb.DBCollection.aggregate(DBCollection.java:1347)
at org.mongodb.morphia.aggregation.AggregationPipelineImpl.aggregate(AggregationPipelineImpl.java:230)
at org.mongodb.morphia.aggregation.AggregationPipelineImpl.aggregate(AggregationPipelineImpl.java:222)
at org.mongodb.morphia.aggregation.AggregationPipelineImpl.aggregate(AggregationPipelineImpl.java:212)

[parvanov]

Also the $let expression takes an object.

[evanchooly]

This feels like a second issue. If so, please open a new one so it can be tracked properly. Thanks.