How to compare a field with another in Mongodb aggregation? -

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I have several levels in the pipeline, now I would like to compare two areas as follows: 

  db.collection.aggregate ({...}, {$ match: {firstfield: {$ gte: "$ secondfield"}}})   
 How to write this question? Close, but the actual greeting is slightly different:  
 {code> db.collection.aggregate ({{" first project ": {" firstfield ": 1," secondfield ": 1," difference ": {" $ gte ": [" $ firstfield " , "$ secondfield"]}}}, {"$ match": {"difference": true}}]   
 for a slightly different invitation, (or possibly) On field comparison, a  true | False  the value can be returned.  
 This allows you to filter later with the phase Pipeline    to present the "Include Include" results depending on the logical state.

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