Update MongoDB field using value of another field

Question

In MongoDB  is it possible to update the value of a field using the value from another field   The equivalent SQL would be something like   UPDATE Person SET Name   FirstName         LastName   And the MongoDB pseudo-code would be   db person update         set     name   firstName         lastName

User · Answer

Regarding this answer  the snapshot function is deprecated in version 3 6  according to this update  So  on version 3 6 and above  it is possible to perform the operation this way   db person find   forEach      function  elem            db person update                                 id  elem  id                                               set                        name  elem firstname         elem lastname

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You should iterate through  For your specific case   db person find   snapshot   forEach      function  elem            db person update                                 id  elem  id                                               set                        name  elem firstname         elem lastname

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For a database with high activity  you may run into issues where your updates affect actively changing records and for this reason I recommend using snapshot     db person find   snapshot   forEach  function  hombre        hombre name   hombre firstName         hombre lastName       db person save hombre          http   docs mongodb org manual reference method cursor snapshot

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The best way to do this is in version 4 2  which allows using of aggregation pipeline in the update document and the updateOne  updateMany or update collection method  Note that the latter has been deprecated in most if not all languages drivers   MongoDB 4 2   Version 4 2 also introduced the  set pipeline stage operator which is an alias for   addFields  I will use  set here as it maps with what we are trying to achieve   db collection  lt update method gt                            set     name       concat      firstName          lastName                MongoDB 3 4   In 3 4  you can use  addFields and the  out aggregation pipeline operators   db collection aggregate                    addFields                   name       concat       firstName          lastName                               out    collection              Note that this does not update your collection but instead replace the existing collection or create a new one  Also for update operations that require  type casting  you will need client side processing  and depending on the operation  you may need to use the find   method instead of the  aggreate   method   MongoDB 3 2 and 3 0  The way we do this is by  projecting our documents and use the  concat string aggregation operator to return the concatenated string  we From there  you then iterate the cursor and use the  set update operator to add the new field to your documents using bulk operations for maximum efficiency   Aggregation query   var cursor   db collection aggregate            project                name       concat       firstName          lastName                   MongoDB 3 2 or newer  from this  you need to use the bulkWrite method   var requests       cursor forEach document   gt         requests push              updateOne                  filter       id   document  id                 update       set      name   document name                           if  requests length     500              Execute per 500 operations and re-init         db collection bulkWrite requests           requests                  if requests length  gt  0         db collection bulkWrite requests       MongoDB 2 6 and  3 0  From this version you need to use the now deprecated Bulk API and its associated methods   var bulk   db collection initializeUnorderedBulkOp    var count   0   cursor snapshot   forEach function document         bulk find     id   document  id    updateOne              set      name   document name               count        if count 500     0               Excecute per 500 operations and re-init         bulk execute            bulk   db collection initializeUnorderedBulkOp                 clean up queues if count  gt  0        bulk execute        MongoDB 2 4  cursor  result   forEach function document        db collection update              id   document  id                 set      name   document name

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Here s what we came up with for copying one field to another for  150 000 records  It took about 6 minutes  but is still significantly less resource intensive than it would have been to instantiate and iterate over the same number of ruby objects   js query          or                  settings mobile notifications       exists   false           settings mobile admin notifications       exists   false                 js for each     function user      if   user settings hasOwnProperty  mobile notifications          user settings mobile notifications   user settings email notifications        if   user settings hasOwnProperty  mobile admin notifications          user settings mobile admin notifications   user settings email admin notifications        db users save user       js    db users find   js query   forEach   js for each     Mongoid  Sessions default command   eval    gt  js

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I would have posted this as a comment  but couldn t  For anyone who lands here trying to update one field using another in the document with the c  driver    I could not figure out how to use any of the UpdateXXX methods and their associated overloads since they take an UpdateDefinition as an argument     we want to set Prop1 to Prop2 class Foo   public string Prop1   get  set    public string Prop2   get  set       void Test           var update   new UpdateDefinitionBuilder lt Foo gt          update Set x   gt  x Prop1   lt new value  no way to get a hold of the object that I can find gt      As a workaround  I found that you can use the RunCommand method on an IMongoDatabase  https   docs mongodb com manual reference command update  dbcmd update   var command   new BsonDocument                          quot update quot    quot CollectionToUpdate quot                    quot updates quot   new BsonArray                                             new BsonDocument                                                         Any filter  here the check is if Prop1 does not exist                                quot q quot   new BsonDocument    quot Prop1 quot     new BsonDocument  quot  exists quot   false                                      set it to the value of Prop2                                quot u quot   new BsonArray   new BsonDocument     quot  set quot     new BsonDocument  quot Prop1 quot    quot  Prop2 quot                                       quot multi quot   true                                                                          database RunCommand lt BsonDocument gt  command

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I tried the above solution but I found it unsuitable for large amounts of data  I then discovered the stream feature   MongoClient connect        function err  db       var c   db collection  yourCollection        var s   c find     your query      stream        s on  data   function doc           c update   id  doc  id     set   name   doc firstName         doc lastName    function err  result       result    true                    s on  end   function               stream can end before all your updates do if you have a lot

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Starting Mongo 4 2  db collection update   can accept an aggregation pipeline  finally allowing the update creation of a field based on another field        firstName   Hello   lastName   World    db collection update              set    name     concat      firstName          lastName                multi  true           firstName     Hello    lastName     World    name     Hello World       The first part    is the match query  filtering which documents to update  in our case all documents   The second part     set    name             is the update aggregation pipeline  note the squared brackets signifying the use of an aggregation pipeline    set is a new aggregation operator and an alias of  addFields  Don t forget   multi  true    otherwise only the first matching document will be updated

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With MongoDB version 4 2   updates are more flexible as it allows the use of aggregation pipeline in its update  updateOne and updateMany  You can now transform your documents using the aggregation operators then update without the need to explicity state the  set command  instead we use  replaceRoot   newRoot     ROOT     Here we use the aggregate query to extract the timestamp from MongoDB s ObjectID   id  field and update the documents  I am not an expert in SQL but I think SQL does not provide any auto generated ObjectID that has timestamp to it  you would have to automatically create that date    var collection    person   agg query                     addFields                    last updated                        toDate       id                                                replaceRoot                newRoot     ROOT                      db getCollection collection  updateMany     agg query   upsert  true

User · Answer

Apparently there is a way to do this efficiently since MongoDB 3 4  see styvane s answer     Obsolete answer below  You cannot refer to the document itself in an update  yet   You ll need to iterate through the documents and update each document using a function  See this answer for an example  or this one for server-side eval

[mongodb] Update MongoDB field using value of another field

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