Question

I saw how this could be accomplished from the mongo shell here: MongoDB: Updating documents using data from the same document

However, I cannot figure out how to do this using the python driver. My goal is thus (in MySQL equivalent):

UPDATE coll SET field1 = field1 + field2;

Answer 1

So far, I found that the easiest way was to adapt the linked SO answer using the pymongo.code.Code class with db.eval(), something like:

db.eval(Code("function () {"
             "coll.find({}, {field1: 1, field2: 2})"
             ".forEach( function(doc) {"
             "    doc.field1 += doc.field2;"
             "    coll.save(doc);"
             "    });"
             "}"))

You can optionally save the js script on the server from the mongo shell with db.system.js.save({_id:'myfunc', value: function(){...}});
and execute from python with db.eval(Code('return myfunc()'))