如何从不同的表中获取特定的列 ASP.NET 核心和实体框架核心

Question

I Want to get some specific column from two tables different ChatMessage And ChatGroup. So that after returning data only contain messageId, Message, SenderUserId, CreatedDate, ChatGroupId.

The query returns all the information of the ApplicationUser and ChatGroup data including the email, hashed Password, user1, user2, again ChatMessages and many more I don't want. So I tried by adding Select in implementation.

A error occur will Implementing this.

Implementation:

public  IEnumerable<ChatMessage> GetMessageInChat(ChatModel model)
{
    try
    {
        PutReadZero(model).Wait();
        return _context.ChatMessages.Where(userID => userID.ChatGroup.Id == model.GroupId)
            .OrderByDescending(m => m.CreatedDate)
            .AsEnumerable()
            .Reverse()
            .Select(p => new 
            {
                p.Id,
                p.Message,
                SenderUserId= p.SenderUser.Id,
                p.CreatedDate,
                ChatGroupId = p.ChatGroup.Id,
            })
            .ToList();
    }
    catch (Exception ex)
    {
        throw new NotImplementedException();
    }
}

ChatMessage.cs:

namespace ProjectName.Models
{
    public class ChatMessage
    {
        public int Id { get; set; }
        public string Message { get; set; }
        public string FileName { get; set; }
        public DateTime CreatedDate { get; set; }
        public ApplicationUser SenderUser { get; set; }
        public ChatGroup ChatGroup { get; set; }
    }
}

ChatGroup:

namespace ProjectName.Models
{
    public class ChatGroup
    {
        public int Id { get; set; }
        public virtual ApplicationUser User1 { get; set; }
        public virtual ApplicationUser User2 { get; set; }
        public int User1Unread { get; set; }
        public int User2Unread { get; set; }
        public DateTime CreatedDate { get; set; }
        public virtual IEnumerable<ChatMessage> ChatMessages { get; set; }
    }
}

ChatModel.cs:

namespace ProjectName
{
    public class ChatModel
    {
        public string SenderId { get; set; }
        public string ReceiverId { get; set; }
        public string Message { get; set; }
        public int GroupId { get; set; }
        public DateTime CreatedDate { get; set; }
        public IFormFile FileContent { get; set; }
    }
}

Error:

Cannot implicitly convert type 'System.Collections.Generic.List<<anonymous type: int Id, string Message, string SenderUserId, System.DateTime CreatedDate, int ChatGroupId>>' to 'System.Collections.Generic.IEnumerable<ProjectName.Models.ChatMessage>'. An explicit conversion exists (are you missing a cast?)

Answer 1

That's because you specified your method should return an IEnumerable<ChatMessage> but your result is returning an anonymous type. If this is an action method in a controller, simply return an IActionResult which allows any data type in its response

public IActionResult GetMessageInChat(ChatModel model)
{
   ... 
    var result = _context.ChatMessages.Where(userID => userID.ChatGroup.Id == model.GroupId).OrderByDescending(m => m.CreatedDate).AsEnumerable().Reverse().Select(p => new { p.Id, p.Message, SenderUserId= p.SenderUser.Id, p.CreatedDate, ChatGroupId = p.ChatGroup.Id, }).ToList();
    return Ok(result);
} 

If it is not, remove the .Select(...) from your query in the original method.

return _context.ChatMessages.Where(userID => userID.ChatGroup.Id == model.GroupId) .OrderByDescending(m => m.CreatedDate).Reverse().ToList();

Then do the filtering with Select in the action method that calls the GetMessageInChat() . The action method should return an IActionResult

Answer 2

You can use FromSqlRaw and write the select query if you want only the data of table without any joins.

`var data = _context.ChatMessages.FromSqlRaw("SELECT * from ChatMessages Where ChatGroupId = model.GroupId").ToList();`

In this way you will get all your required data

Answer 3

Have a new model that only contains the properties you want.

public class ChatMessageViewModel
{
    public int Id { get; set; }
    public string Message { get; set; }
    public int SenderUserId { get; set; }
    public int ChatGroupId { get; set; }
    public DateTime CreatedDate { get; set; }
}

Then select this model in the linq:

.Select(p => new ChatMessageViewModel
{
    Id = p.Id,
    Message = p.Message,
    SenderUserId= p.SenderUser.Id,
    CreatedDate  = p.CreatedDate,
    ChatGroupId = p.ChatGroup.Id,
})

Then change the return type to IEnumerable<ChatMessageViewModel>

Answer 4

If you need strongly typed object from the GetMessageInChat method, then you should just go with the answer of @mj1313.

But if you don't need a typed object (for example, if GetMessageInChat is an action method in some controller class), you can simply change the method signature to return a dynamic type -

public dynamic GetMessageInChat(ChatModel model)

You can call the method as -

var myList = myObject.GetMessageInChat(chatModel);