Ordering fields from find query with projection

user_688355 0
Viewed 9855

I have a Mongo find query that works well to extract specific fields from a large document like...

db.profiles.find(
  { "profile.ModelID" : 'LZ241M4' },
  {
    _id : 0,
    "profile.ModelID" : 1,
    "profile.AVersion" : 2,
    "profile.SVersion" : 3
  }
);

...this produces the following output. Note how the SVersion comes before the AVersion in the document even though my projection asked for AVersion before SVersion.

{ "profile" : { "ModelID" : "LZ241M4", "SVersion" : "3.5", "AVersion" : "4.0.3" } }
{ "profile" : { "ModelID" : "LZ241M4", "SVersion" : "4.0", "AVersion" : "4.0.3" } }

...the problem is that I want the output to be...

{ "profile" : { "ModelID" : "LZ241M4", "AVersion" : "4.0.3", "SVersion" : "3.5" } }
{ "profile" : { "ModelID" : "LZ241M4", "AVersion" : "4.0.3", "SVersion" : "4.0" } }

What do I have to do get the Mongo JavaScript shell to present the results of my query in the field order that I specify?

mongodb
4 Answers

I have achieved it by projecting the fields using aliases, instead of including and excluding by 0 and 1s. Try this:

{
_id : 0,      
"profile.ModelID" :"$profile.ModelID", 
"profile.AVersion":"$profile.AVersion",
"profile.SVersion":"$profile.SVersion"
}
user_89541050

Another solution I applied to achieve this is the following:

db.profiles
  .find({ "profile.ModelID" : 'LZ241M4' })
  .toArray()
  .map(doc => ({
    profile: {
      ModelID: doc.profile.ModelID,
      AVersion: doc.profile.AVersion,
      SVersion: doc.profile.SVersion
    }
  }))
user_5922540

Since version 2.6 (that came out in 2014) MongoDB preserves the order of the document fields following the write operation (source).

P.S. If you are using Python you might find this interesting.

user_54407410
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