在ASP.NET Web API 2上使用Linq筛选IEnumerable Collection

Question

I need to filter my data based on a Boolean condition using Linq on ASP.NET Web Api 2 C#. But my JSON data returns null when I try filtering the data.

The data is a List of Invites, I want to filter the results based on a MemberID and if the user is attending(IsAttending)

Here is the Member Model

public class Member
{
    public int MemberID { get; set; }
    public string MemberName { get; set; }
    public bool IsAdmin { get; set; }
    public bool IsAttending { get; set; }
    public int MemberNumber { get; set; }
    public string PhoneNumber { get; set; }
    public int InviteID { get; set; }
    [JsonIgnore]
    public virtual Invite Iinvite { get; set; }
}

InviteModel

public class Invite
{
    public int InviteID { get; set; }
    public string Subject { get; set; }
    public string DateTime { get; set; }
    public virtual List<Member> Members { get; set; }
}

Original JSON data without filtering

[{
    "InviteID": 1,
    "Subject": "Hell",
    "DateTime": "March 2017",
    "Members": [{
        "MemberID": 1,
        "MemberName": "roger",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 1
    },
    {
        "MemberID": 2,
        "MemberName": "nkosi",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 1
    },
    {
        "MemberID": 3,
        "MemberName": "Mphile",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 1
    }]
},
{
    "InviteID": 2,
    "Subject": "Hell",
    "DateTime": "March 2018",
    "Members": [{
        "MemberID": 4,
        "MemberName": "roger",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 2
    },
    {
        "MemberID": 5,
        "MemberName": "nkosi",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 2
    },
    {
        "MemberID": 6,
        "MemberName": "Mphile",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 2
    }]
},
{
    "InviteID": 3,
    "Subject": "Hell",
    "DateTime": "Marchs 2017",
    "Members": [{
        "MemberID": 7,
        "MemberName": "Roger Nkosi",
        "IsAdmin": false,
        "IsAttending": true,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 3
    },
    {
        "MemberID": 8,
        "MemberName": "nkosi",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 3
    },
    {
        "MemberID": 9,
        "MemberName": "Mphile",
        "IsAdmin": false,
        "IsAttending": false,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 3
    }]
},
{
    "InviteID": 4,
    "Subject": "Hell",
    "DateTime": "Marchs 2017",
    "Members": [{
        "MemberID": 10,
        "MemberName": "Roger Nkosi",
        "IsAdmin": false,
        "IsAttending": true,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 4
    },
    {
        "MemberID": 11,
        "MemberName": "nkosi",
        "IsAdmin": false,
        "IsAttending": true,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 4
    },
    {
        "MemberID": 12,
        "MemberName": "Mphile",
        "IsAdmin": false,
        "IsAttending": true,
        "MemberNumber": 0,
        "PhoneNumber": null,
        "InviteID": 4
    }]
}]

Here is how I filter and return the data

return db.invites.Include("members").ToList()
                .Where(x => x.Members == db.members
                                .Where(xx => xx.IsAttending == true && xx.MemberID == 4)).ToList();

Answer 1

What you want is not impossible, is just not achievable with a single LINQ query to the DB.

Assuming memberId contains the id of the member you want to filter on:

var invites = db.invites.Include(i => i.Members).Where(i => i.Members.Any(m => m.IsAttending && m.MemberID == memberId)).ToList(); // db query
invites = invites.Select(i =>
    new Invite
    {
        DateTime = i.DateTime,
        InviteID = i.InviteID,
        Subject = i.Subject,
        Members = i.Members.Where(m => m.IsAttending && m.MemberID == memberId).ToList()
    }).ToList(); // this removes other members (in memory)

 return invites;

Answer 2

You should use the Linq Any() operator.

It will look something more like

return db.invites.Include("members").Where(x => x.Members.Any(xx => xx.IsAttending && xx.MemberID == 4).ToList()

By the way, I removed the ToList() after db.invites because that will cause the entire members collection to enumerate. Not sure how big your database is, but that would eventually be a problem.