using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity.Core.EntityClient;
using System.Data.Entity.Core.Metadata.Edm;
using System.Data.Entity.Core.Objects;
using System.Data.Entity.Core.Objects.DataClasses;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;

namespace Repository
{
    public partial class EfEntities
    {
        public EfEntities(string connectionString)
            : base(connectionString)
        {

        }

        public static Entities NewEntities()
        {
            return new Entities(GenerateEFConnection());

        }

        public static string GenerateEFConnection()
        {
            return new EntityConnectionStringBuilder
            {
                Metadata = "res://*/",
                ProviderConnectionString = YOUR_CONNECTION_STRING,
                Provider = "System.Data.SqlClient"
            }.ToString(); 
        }

        public override int SaveChanges()
        {
            //Figure How many keys of each type we need, so we can grab them from db all at once 
            //Request keys from DB 
            var keys = GenerateKeys(this.ChangeTracker.Entries().LongCount(e => e.Entity is ISequenceBase && e.State == EntityState.Added));

            ObjectContext objectContext = ((IObjectContextAdapter)this).ObjectContext;
            var w = objectContext.MetadataWorkspace;

            foreach (var entry in this.ChangeTracker.Entries().Where(e => e.Entity is ISequenceBase
                && e.State == EntityState.Added))
            {
                var entityType = entry.Entity.GetType();
                var entityName = entityType.Name;
                var wKey = w.GetEntityContainer(objectContext.DefaultContainerName, DataSpace.CSpace)
                    .BaseEntitySets.First(meta => meta.ElementType.Name == entityName)
                    .ElementType.KeyMembers.Select(k => k.Name).FirstOrDefault();

                if (!string.IsNullOrEmpty(wKey))
                    entry.Property(wKey).CurrentValue = keys.Dequeue();
            }

            int result = base.SaveChanges();
            return result;
        }

        private Queue<long> GenerateKeys(long sequenceKeysRequested)
        {
            Queue<long> sequences = null;
            if (sequenceKeysRequested > 0)
            {
                var paramSequenceObject = new SqlParameter("@sequence_name", SqlDbType.VarChar) {Value = "{Your Sequence Object}"};
                var paramRangeSize = new SqlParameter("@range_size", SqlDbType.Int) {Value = sequenceKeysRequested};
                var paramRangeFirstValue = new SqlParameter("@range_first_value", SqlDbType.Variant){ Direction = ParameterDirection.Output};
                var paramRangeLastValue = new SqlParameter("@range_last_value", SqlDbType.Variant) { Direction = ParameterDirection.Output };
                this.Database.ExecuteSqlCommand("exec sys.sp_sequence_get_range @sequence_name, @range_size, @range_first_value OUTPUT, @range_last_value OUTPUT", paramSequenceObject, paramRangeSize, paramRangeFirstValue, paramRangeLastValue);

                sequences = new Queue<long>((int) ((long) paramRangeLastValue.Value - (long) paramRangeFirstValue.Value)+1);
                for (long i = (long) paramRangeFirstValue.Value; i <= (long) paramRangeLastValue.Value; i++)
                {
                    sequences.Enqueue(i);
                }

            }
            return sequences;
        }
    }
}