fix #2992 throw a better error message when Join<TParent, TChild>() is called multiple times with different children

Author: Mpdreamz

src/Nest/Mapping/Types/Core/Join/Relations.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using Newtonsoft.Json;
@@ -15,9 +16,19 @@ public Relations(Dictionary<RelationName, Children> container)
 			: base(container.Select(kv => kv).ToDictionary(kv => kv.Key, kv => kv.Value))
 		{ }
 
-		public void Add(RelationName type, Children children) => BackingDictionary.Add(type, children);
-		public void Add(RelationName type, RelationName child, params RelationName[] moreChildren) =>
+		public void Add(RelationName type, Children children)
+		{
+			if (BackingDictionary.ContainsKey(type))
+				throw new ArgumentException($"{type} is already mapped as parent, you have to map all it's children as a single entry");
+			BackingDictionary.Add(type, children);
+		}
+
+		public void Add(RelationName type, RelationName child, params RelationName[] moreChildren)
+		{
+			if (BackingDictionary.ContainsKey(type))
+				throw new ArgumentException($"{type} is already mapped as parent, you have to map all it's children as a single entry");
 			BackingDictionary.Add(type, new Children(child, moreChildren));
+		}
 	}
 
 	public class RelationsDescriptor : IsADictionaryDescriptorBase<RelationsDescriptor,	IRelations, RelationName, Children>
@@ -27,8 +38,20 @@ internal RelationsDescriptor(IRelations relations) : base(relations) { }
 
 		public RelationsDescriptor Join(RelationName parent, RelationName child, params RelationName[] moreChildren) =>
 			Assign(parent, new Children(child, moreChildren));
-		public RelationsDescriptor Join<TParent>(RelationName child, params RelationName[] moreChildren) =>
-			Assign(typeof(TParent), new Children(child, moreChildren));
-		public RelationsDescriptor Join<TParent, TChild>() => Assign(typeof(TParent), typeof(TChild));
+
+		public RelationsDescriptor Join<TParent>(RelationName child, params RelationName[] moreChildren)
+		{
+			if (this.PromisedValue.ContainsKey(typeof(TParent))) throw new ArgumentException(Message(typeof(TParent)));
+			return Assign(typeof(TParent), new Children(child, moreChildren));
+		}
+
+		public RelationsDescriptor Join<TParent, TChild>()
+		{
+			if (this.PromisedValue.ContainsKey(typeof(TParent))) throw new ArgumentException(Message(typeof(TParent)));
+			return Assign(typeof(TParent), typeof(TChild));
+		}
+
+		private static string Message(Type t) =>
+			$"{t.Name} is already mapped. Use Join<TParent>(typeof(ChildA), typeof(ChildB), ..) to add multiple children in one go";
 	}
 }

src/Tests/ClientConcepts/HighLevel/Mapping/ParentChildJoins.doc.cs

@@ -72,6 +72,7 @@ public void SimpleParentChildMapping()
 								.Name(p=>p.MyJoinField)
 								.Relations(r => r
 									.Join<MyDocument, MyChild>()
+									.Join<MyDocument>(typeof(MyChild), typeof(MyChild))
 								)
 							)
 						)