MongoDB copy a field to another collection with a foreign key -

i want copy on color user collection car collection. i'm using foreign key userid.

> db.test1.user.find() { "_id" : objectid("515f7db83f71d6bcb1c41a48"), "age" : 33, "color" : "blue" } { "_id" : objectid("515f7dc03f71d6bcb1c41a49"), "age" : 52, "color" : "red" } { "_id" : objectid("515f7dc43f71d6bcb1c41a4a"), "age" : 43, "color" : "yellow" }   > db.test2.car.find() { "_id" : objectid("515f84883f71d6bcb1c41a54"), "speed" : 291, "userid" : objectid("515f7db83f71d6bcb1c41a48") } { "_id" : objectid("515f84883f71d6bcb1c41a55"), "speed" : 202, "userid" : objectid("515f7db83f71d6bcb1c41a49") } { "_id" : objectid("515f84883f71d6bcb1c41a56"), "speed" : 193, "userid" : objectid("515f7db83f71d6bcb1c41a4a") } 

here query

db.test1.user.find().foreach(  function(x) {   db.test2.car.update( { userid: x._id }, { $set: { color: x.color} } )  } ); 

i want result:

> db.test2.car.find() { "_id" : objectid("515f84883f71d6bcb1c41a54"), "speed" : 291, "userid" : objectid("515f7db83f71d6bcb1c41a48"), "color" : "blue" } { "_id" : objectid("515f84883f71d6bcb1c41a55"), "speed" : 202, "userid" : objectid("515f7db83f71d6bcb1c41a49"), "color" : "red" } { "_id" : objectid("515f84883f71d6bcb1c41a56"), "speed" : 193, "userid" : objectid("515f7db83f71d6bcb1c41a4a"), "color" : "yellow" } 

thanks !

there several issues test set up:

• case of field names not match (you referencing color instead of color when copying)
• only 1 of example foreign keys matches in target collection: objectid('515f7db83f71d6bcb1c41a48')
• your update affect first matching document "foreign key". fine 1:1 relationship, not 1:many

a corrected example taking above account (aside non-matching keys):

db.test1.user.find().foreach(      function(x) {         db.test2.car.update(             // query              { userid: x._id },              // update              { $set: { color: x.color} },              // options:             { "multi" : true } // update matching documents         );     } ); 

which results in setting {color:blue} foreign key matches in sample documents:

db.test2.car.find() {     "_id" : objectid("515f84883f71d6bcb1c41a55"),     "speed" : 202,     "userid" : objectid("515f7db83f71d6bcb1c41a49") } {     "_id" : objectid("515f84883f71d6bcb1c41a56"),     "speed" : 193,     "userid" : objectid("515f7db83f71d6bcb1c41a4a") } {     "_id" : objectid("515f84883f71d6bcb1c41a54"),     "color" : "blue",     "speed" : 291,     "userid" : objectid("515f7db83f71d6bcb1c41a48") }