Add several fields into an array containing objects

Question

I am trying to update an object into an array in MongoDB.
Here are my documents :

> db.devis.insert({id:1,obj:[{'code':'A'},{'code':'B'}]})  
> db.devis.insert({id:2,obj:[{'code':'C'},{'code':'D'}]})  

> db.devis.find()
[
  {
    id: 1,
    obj: [ { code: 'A' }, { code: 'B' } ]
  },
  {
    id: 2,
    obj: [ { code: 'C'}, { code: 'D' } ]
  }
]

I select the document with the code 'B'.
I can add one field with this command :

> db.devis.update({'obj.code':'B'},{$set: {'obj.$.a':'a'}})  

# Result
{
  id: 1,
  obj: [ { code: 'A'}, { code: 'B', a: 'a'} ]
}

Now I want to add several fields :

> var = {'foo':'foo', 'bar':'bar'}
> db.devis.update({'obj.code':'B'},{$set: {'obj.$': var}})

# Result
{
  id: 1,
  obj: [ { code: 'A'}, { foo: 'foo', bar: 'bar'} ]
}

# What I want
{
  id: 1,
  obj: [ { code: 'A'}, { code: 'B', foo: 'foo', bar: 'bar'} ]
}

The object is overwritten.
It is possible to insert in it several fields from a variable ?

Clarity: You want to keep code but get rid of a and add foo and bar? — Buzz Moschetti
– Buzz Moschetti, Commented Nov 10, 2020 at 12:16
No the a was just an example to add one field, @J.F. answered exactly what I was looking for. — Martin Allusse
– Martin Allusse, Commented Nov 10, 2020 at 12:33

J.F. · Accepted Answer · 2020-11-10 12:18:33Z

The problem here is you are seting the entire object.

When you do {$set: {obj.$:{yourObject}}} you are telling mongo: "Set entire object obj as the object I have passed".

Look at this example

The object with code : B is completly new because your $ operator point at the object and $set will put the new object at this position.

You need this:

db.collection.update({
  "obj.code": "B"
},
{
  "$set": {
    "obj.$.foo": "bar",
    "obj.$.foo2": "bar2"
  }
})

In this case, your $set uses the $ operator to update the field into the object, no the object itself.
In this way you are telling mongo: "Put into the object you have found a variable called 'foo' with value 'bar' and another variable 'foo2' with value 'bar2'"

I hope I have explained well to note the difference between the two queries.

So, no tested, but using:

var = {'obj.$.foo':'foo', 'obj.$.bar':'bar'}
db.devis.update({'obj.code':'B'},{$set: var})

Should works.

Example here

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