update one element of array of objects in mongodb

Question

I have the following documents:

{
  _id: ObjectId('111111111111111111114444'),
  books: [
    {
      id: ObjectId('111111111111111111113333'),
      pictures: []
    },
    {
      id: ObjectId('111111111111111111112222'),
      pictures: []
    }
  ],
  // others attributes
}

What I need to do is to add an element to pictures. Keep in mind that I have the documentId which is the id of the document, and the bookId which is the Id of an element of books.

If I want to add the element {description: "nice picture"} to the document 111111111111111111114444 in the book 111111111111111111113333, the updated document would look like

{
  _id: ObjectId('111111111111111111114444'),
  books: [
    {
      id: ObjectId('111111111111111111113333'),
      pictures: [
        {
          description: "nice picture"
        }
      ]
    },
    {
      id: ObjectId('111111111111111111112222'),
      pictures: []
    }
  ],
  // others attributes
}

The books is an array of objects, which yes, they have an id — Loudrous
– Loudrous, Commented Nov 4, 2021 at 12:59

Takis · Accepted Answer · 2022-04-09 16:01:58Z

1

Query

pipeline update requires MongoDB >= 4.2
filter the "_id"
map on books if id=2 merge objects, with the pictures containing the new element
else if not id=2 dont change the book

*i used 1,2,3 for ids (its the same code)

PlayMongo

update({"_id" : 1},
[{"$set": 
   {"books": 
     {"$map": 
       {"input": "$books",
        "in": 
        {"$cond": 
          [{"$eq": ["$$book.id", 2]},
           {"$mergeObjects": 
             ["$$book",
               {"pictures": 
                 {"$concatArrays": 
                   ["$$book.pictures", [{"description": "nice picture"}]]}}]},
           "$$book"]},
        "as": "book"}}}}])

Edit

Query

if pictures isnt't array(or dont exists), creates an empty array

PlayMongo

update({"_id" : 1},
[{"$set": 
    {"books": 
      {"$map": 
        {"input": "$books",
          "in": 
          {"$cond": 
            [{"$eq": ["$$book.id", 2]},
              {"$mergeObjects": 
                ["$$book",
                  {"pictures": 
                    {"$concatArrays": 
                      [{"$cond": 
                          [{"$isArray": ["$$book.pictures"]}, "$$book.pictures",
                            []]},
                        [{"description": "nice picture"}]]}}]},
              "$$book"]},
          "as": "book"}}}}])

edited Apr 9, 2022 at 16:01

answered Nov 4, 2021 at 13:08

Takis

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4 Comments

Loudrous Over a year ago

what if there is not an array yet? It would make concatArrays of null and another array which probably will turn out as null. We can't make $concatArrays of undefined. How can I solve this? I tried with $push but for some reason it doesn't accept it..

Takis Over a year ago

i updated for that case but if you have special cases of schema, ask about them in the question, because answer is based on the schema you sended that pictures was array always.

Loudrous Over a year ago

I apologize for my mistake. Thank you

Takis Over a year ago

no worries its mostly for you to get better answers

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