How to properly map to existing collections?

[rm-code]

Let's say I have a situation where an incoming collections needs to be mapped onto an existing collection. src.Adapt(dest) doesn't produce the intended outcome since (I assume) Mapster doesn't know how to match the items in the collection.

using Mapster;
using MapsterTest;
using System.Text.Json;

var collection = new List<NestedCollectionEntity> {
  new NestedCollectionEntity
  {
    Id = 1,
    Value = "NestedCollectionEntity",
    CreatedOn = DateTime.Now,
    CreatedBy = "Me"
  }
};
var dtoCollection = new List<NestedCollectionDto> {
  new NestedCollectionDto
  {
    Id = 1,
    Value = "NestedCollectionDto",
  },
  new NestedCollectionDto
  {
    Id = 2,
    Value = "NestedCollectionDto",
  }
};

Console.WriteLine(JsonSerializer.Serialize(collection, new JsonSerializerOptions { WriteIndented = true }));

dtoCollection.Adapt(collection);

Console.WriteLine("After Mapping");
Console.WriteLine(JsonSerializer.Serialize(collection, new JsonSerializerOptions { WriteIndented = true }));

The output this produces looks like this:

[
  {
    "Id": 1,
    "Value": "NestedCollectionEntity",
    "CreatedOn": "2023-02-17T09:43:38.8157027+01:00",
    "CreatedBy": "Me"
  }
]

After Mapping
[
  {
    "Id": 1,
    "Value": "NestedCollectionDto",
    "CreatedOn": "0001-01-01T00:00:00",
    "CreatedBy": null
  },
  {
    "Id": 2,
    "Value": "NestedCollectionDto",
    "CreatedOn": "0001-01-01T00:00:00",
    "CreatedBy": null
  }
]

Is there an easy way to set this up via Mapster?

My current workaround is to write my own "matching logic":

foreach(var dto in dtoCollection)
{
  var match = collection.FirstOrDefault(o => o.Id == dto.Id);
  if (match != null)
  {
    dto.Adapt(match);
  }
  else
  {
    var n = dto.Adapt<NestedCollectionEntity>();
    collection.Add(n);
  }
}

[andrerav]

Possible duplicate of #537

[DocSvartz]

Hello, The collection adapter does not interact with elements of the collection TDistination Its logic For is approximately as follows:

 var _result = new Collection<TDistination>();

 foreach (var item in Collection<TSourse>)
 {
     _result.Add(item.Adapt<TDistination>());
 }

 return _result;

You converted NestedCollectionDto => NestedCollectionEntity

Since the specified fields with such names in "CreatedOn" "CreatedBy" in NestedCollectionDto was not found in them they were assigned default values

[DocSvartz]

Hello, looks like you wanted to get .unionby() or .intersectby() between List Tsource and List TDistination ?