You can't find a column in your mongodb data source, but you think its there? 

Please check first, if it really exists. Please open your favorite MongoDB client and type the following to find "myElement"

db.collection.find( {myElement: { $exists: true } } )

If this is the case, the column was added recently and is not present in the first rows, so you have to adjust your data source Parameters, and increase the mongodb.maxRowsNumber model property. Please refer to 13.9 MongoDB Connector in the Reference Guide for more information.

To find out how many rows you have in your MongoDB collection type

db.collection.count()