using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var records = new List<Foo>() {

            new Foo(1, 1),

            new Foo(2, 1),

            new Foo(3, 1),

            new Foo(4, 2),

            new Foo(5, 2),

            new Foo(6, 3),

            new Foo(7, 4),

            new Foo(8, 5),

            new Foo(9, 6),

            new Foo(10, 7),

        };

        const int maxPerGroup = 3;

        var groupAssigns = records

            // Find and group all the Foo's that cannot be in the same collection.

            .GroupBy(r => r.GroupId)

            // For each Foo with the same Group id, assign an index by its position.

            // This will be used in the next step to assign same-groupid foos to different buckets.

            .SelectMany(g => g.Select((r, i) => new {

                Bucket = i,

                Record = r

            }))

            // As mentioned above, assign phobic Foos to different buckets.

            .GroupBy(g => g.Bucket, p => p.Record)

            // Now take each bucket and split it into members that fit and overflows, if any.

            .Select(b => b.Chunk(maxPerGroup).ToArray())

            .Select(b => new {

                Group = b[0],

                Overflows = b.Skip(1).SelectMany(c => c)

            })

            .Aggregate(

                new { Groups = new List<List<Foo>>(), Overflows = new Queue<Foo>()},

                (acc, next) => {

                    acc.Groups.Add(next.Group.ToList());

                    foreach(var o in next.Overflows)

                        acc.Overflows.Enqueue(o);

                    return acc;

                }

            );

        var groups = groupAssigns.Groups;

        foreach(var g in groups)

        {

            while(g.Count() < maxPerGroup && groupAssigns.Overflows.Count > 0)

                g.Add(groupAssigns.Overflows.Dequeue());

        }

        groups.AddRange(groupAssigns.Overflows.Chunk(maxPerGroup).Select(c => c.ToList()));

​

​

        for (int i = 0; i < groups.Count(); ++i) {

            System.Console.WriteLine($"Group {i}: {Newtonsoft.Json.JsonConvert.SerializeObject(groups[i])}");

        }

    }

    public class Foo {

        public int Id;

        public int GroupId;

        public Foo(int id, int groupId) {

            this.Id = id;

            this.GroupId = groupId;

        }

    }

}