how I can send 2 object to my API (ASP.NET)?

I have a problem that I cannot solve. I would like to transmit the deck created by the user to my API but the problem is that the card and the deck are two different entities in my database that’s why I need to pass the information of the deck and the list card to my API to add them to my database.

my entity:

    public class Card
        {
    
            public Card() {
                this.Decks = new HashSet<Deck>();
            }
    
            public int Id { get; set; }
    
            [Column(TypeName = "json")]
            public string Content { get; set; }
    
            [NotMapped]
            public virtual ICollection<Deck> Decks { get; set; }
        }

public class Deck
    {

        public Deck() {
            this.Cards = new HashSet<Card>();
        }

        public int Id { get; set; }
        public string Name { get; set; }

        [DataType(DataType.Date)]
        public DateTime CreateAt { get; set; }

        public User User { get; set; }
        [NotMapped]
        public virtual ICollection<Card> Cards { get; set; }
    }

public class Join
    {
        public int DeckId { get; set; }
        public Deck Deck { get; set; }
        public int CardId { get; set; }
        public Card Card { get; set; }
    }

my API:

[HttpPost]
public void Add ([FromBody] JsonObject request) {
            
}

the JSON:

{ 
    "Deck":{
        "Name": "", 
        "CreateAt": "2007-07-15", 
        "User": "null"
    },
    "Crads":[
        {"content": {}}, 
        {"content": {}} 
    ]
}

Response:

{
    "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
    "title": "One or more validation errors occurred.",
    "status": 400,
    "traceId": "|99e854f-4f63859a690203b6.",
    "errors": {
        "$.Deck.User": [
            "The JSON value could not be converted to MTG_Deck.Models.User. Path: $.Deck.User | LineNumber: 4 | BytePositionInLine: 22."
        ]
    }
}

Answers:

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Method 1

I think it is better that in your API you receive a DTO that encapsulates the two classes you need (Deck and Card). Inside your method then take the information you need and save the entities correctly in the database.

public class AddDeckAndCardDTO
{
    public Deck Deck { get; set; }
    public List<Card> Card { get; set; }
}

And the method Add in the API

[HttpPost]
public void Add ([FromBody] AddDeckAndCardDTO request) 
 {
     var card = request.Card;
     var deck = request.Deck
        
     // the code to mapped entities
     // the code to save your entities
 }

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