ported PR #2434 over manually because the reindex observable implementation deviates too much (#2435)

Author: Mpdreamz

src/Nest/Document/Multiple/Reindex/ReindexObservable.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using Elasticsearch.Net;
 
 namespace Nest
@@ -43,12 +45,15 @@ private void Reindex(IObserver<IReindexResponse<T>> observer)
 			var resolvedTo = toIndex.Resolve(this._connectionSettings);
 			resolvedTo.ThrowIfNullOrEmpty(nameof(toIndex));
 
-			var indexSettings = this._client.GetIndex(fromIndex);
-			Func<CreateIndexDescriptor, ICreateIndexRequest> settings =  (ci) => this._reindexRequest.CreateIndexRequest ?? ci;
-			var createIndexResponse = this._client.CreateIndex(
-				toIndex, (c) => settings(c.InitializeUsing(indexSettings.Indices[resolvedFrom])));
-			if (!createIndexResponse.IsValid)
-				throw new ElasticsearchClientException(PipelineFailure.BadResponse, $"Failed to create destination index {toIndex}.", createIndexResponse.ApiCall);
+			if (!this._reindexRequest.OmitIndexCreation)
+			{
+				var indexSettings = this._client.GetIndex(fromIndex);
+				Func<CreateIndexDescriptor, ICreateIndexRequest> settings = (ci) => this._reindexRequest.CreateIndexRequest ?? ci;
+				var createIndexResponse = this._client.CreateIndex(
+					toIndex, (c) => settings(c.InitializeUsing(indexSettings.Indices[resolvedFrom])));
+				if (!createIndexResponse.IsValid)
+					throw new ElasticsearchClientException(PipelineFailure.BadResponse, $"Failed to create destination index {toIndex}.", createIndexResponse.ApiCall);
+			}
 
 			var page = 0;
 			var searchResult = this._client.Search<T>(
@@ -58,6 +63,7 @@ private void Reindex(IObserver<IReindexResponse<T>> observer)
 					.From(0)
 					.Size(size)
 					.Query(q=>this._reindexRequest.Query)
+					.Sort(sort=>sort.Field(f=>f.Field("_doc")))
 					.SearchType(SearchType.Scan)
 					.Scroll(scroll)
 				);

src/Nest/Document/Multiple/Reindex/ReindexRequest.cs

@@ -14,11 +14,18 @@ public interface IReindexRequest
 
 		ICreateIndexRequest CreateIndexRequest { get; set; }
 
+		/// <summary>
+		/// Do not send a create index call on <see cref="To"/>, assume the index has been created outside of the reindex.
+		/// </summary>
+		bool OmitIndexCreation { get; set; }
+
+		[Obsolete("Unused property that is removed in 5.0")]
 		IPutMappingRequest PutMappingRequest { get; set; } 
 	}
 
 	public class ReindexRequest : IReindexRequest
 	{
+		public bool OmitIndexCreation { get; set; }
 		public IndexName To { get; set; }
 		public IndexName From { get; set; }
 		public Types Type { get; set; }
@@ -37,6 +44,7 @@ public ReindexRequest(IndexName from, IndexName to, Types type)
 
 	public class ReindexDescriptor<T> : DescriptorBase<ReindexDescriptor<T>, IReindexRequest>, IReindexRequest where T : class
 	{
+		bool IReindexRequest.OmitIndexCreation { get; set; }
 		IndexName IReindexRequest.To { get; set; }
 		IndexName IReindexRequest.From { get; set; }
 		Types IReindexRequest.Type { get; set; }
@@ -77,6 +85,9 @@ public ReindexDescriptor(IndexName from, IndexName to)
 		public ReindexDescriptor<T> Query(Func<QueryContainerDescriptor<T>, QueryContainer> querySelector) =>
 			Assign(a => a.Query = querySelector?.Invoke(new QueryContainerDescriptor<T>()));
 
+		/// <inheritdoc/>
+		public ReindexDescriptor<T> OmitIndexCreation(bool omit = true) => Assign(a => a.OmitIndexCreation = true);
+
 		/// <summary>
 		/// A query to optionally limit the documents to use for the reindex operation.  
 		/// </summary>