MongoDB - How to push an embedded array into an existing document

Question

Model:

[BsonIgnoreExtraElements]
public class UserPortfolioList
{
    public string? Username { get; set; }
    public List<Pflist>? Pflist { get; set; }

}

public class Pflist
{
    public string PfName { get; set; } = "DEFAULT NAME";
    public List<string>? Symbols { get; set; }

}

The existing MongoDB Document:

{
  "username": "aaa",
  "pflist": [
    {
      "pfName": "US TECH A",
      "symbols": [
        "GOOG",
        "MSFT"
      ]
    }
  ]
}

The array I want to push (Same username, but different pfName and symbols)

{
  "username": "aaa",
  "pflist": [
    {
      "pfName": "US TECH C",
      "symbols": [
        "AAPL",
        "TSLA"
      ]
    }
  ]
}

Expected result:

{
  "username": "aaa",
  "pflist": [
    {
      "pfName": "US TECH A",
      "symbols": [
        "GOOG",
        "MSFT"
      ]
    },
    {
      "pfName": "US TECH C",
      "symbols": [
        "AAPL",
        "TSLA"
      ]
    }
  ]
}

Code:

public async Task UpdateUserPfAsync(UserPortfolioList userPflist)

  await _UserPortfoliosCollection.UpdateOneAsync(Builders<UserPortfolioList>.Filter.Eq(
    "Username", userPflist.Username),
    Builders<UserPortfolioList>.Update.Push(x => x.Pflist, userPflist.Pflist));

The compiler throws out an error:

can not convert lambda express to type......

I almost tried every approach on the StackOverflow, but none of them works for me.

Please help. Thank you!

Answer 1

Concern:

Since you are adding a single object to pflist, you should use userPflist.Pflist[0] instead of userPflist.Pflist as with userPflist.Pflist will lead to the result:

{
  username: "aaa",
  pflist: [
    0: Object
    1: Array
      0: Object
  ]
}

---

Push single element to the array

MongoDB query:

db.collection.update({
  username: "aaa"
},
{
  $push: {
    "pflist": {
      "pfName": "US TECH C",
      "symbols": [
        "AAPL",
        "TSLA"
      ]
    }
  }
})

Sample Mongo Playground (Push single element to the array)

MongoDB C#:

await _UserPortfoliosCollection.UpdateOneAsync(
    Builders<UserPortfolioList>.Filter.Eq("Username", userPflist.Username),
    Builders<UserPortfolioList>.Update.Push(x => x.Pflist, userPflist.Pflist[0]));

Note: The lambda expression for .Push() works if the value is an object instead of an array.

---

Push multiple elements to the array

To push multiple elements into the array, you need a $each operator.

MongoDB query:

db.collection.update({
  username: "aaa"
},
{
  $push: {
    "pflist": {
      $each: [
        {
          "pfName": "US TECH C",
          "symbols": [
            "AAPL",
            "TSLA"
          ]
        }
      ]
    }
  }
})

Sample Mongo Playground

MongoDB C#:

await _UserPortfoliosCollection.UpdateOneAsync(
    Builders<UserPortfolioList>.Filter.Eq("Username", userPflist.Username),
    Builders<UserPortfolioList>.Update.PushEach(x => x.Pflist, userPflist.Pflist));

---

Update Result