Prefer to keep existing ways when joining (fixes #1424)

js/id/actions/join.js

@@ -20,6 +20,15 @@ iD.actions.Join = function(ids) {
             b = graph.entity(idB),
             nodes;
 
+        // Prefer to keep an existing way.
+        if (a.isNew() && !b.isNew()) {
+            var tmp = a;
+            a = b;
+            b = tmp;
+            idA = a.id;
+            idB = b.id;
+        }
+
         if (a.first() === b.first()) {
             // a <-- b ==> c
             // Expected result:

test/spec/actions/join.js

@@ -149,6 +149,25 @@ describe("iD.actions.Join", function () {
         expect(graph.entity('-').tags).to.eql({'lanes:backward': 2});
     });
 
+    it("prefers to keep existing ways", function () {
+        // a --> b ==> c
+        // --- is new, === is existing
+        // Expected result:
+        // a ==> b ==> c
+        var graph = iD.Graph({
+                'a': iD.Node({id: 'a'}),
+                'b': iD.Node({id: 'b'}),
+                'c': iD.Node({id: 'c'}),
+                'w-1': iD.Way({id: 'w-1', nodes: ['a', 'b']}),
+                'w1': iD.Way({id: 'w1', nodes: ['b', 'c']})
+            });
+
+        graph = iD.actions.Join(['w-1', 'w1'])(graph);
+
+        expect(graph.entity('w1').nodes).to.eql(['a', 'b', 'c']);
+        expect(graph.hasEntity('w-1')).to.be.undefined;
+    });
+
     it("merges tags", function () {
         var graph = iD.Graph({
                 'a': iD.Node({id: 'a'}),