From 4720a84d38c23de9c007a1e4d0f1c53611d47c4a Mon Sep 17 00:00:00 2001
From: John Firebaugh <john.firebaugh@gmail.com>
Date: Tue, 12 Mar 2013 16:22:10 -0700
Subject: [PATCH] No-op a null tag merge

Reduces unnecessary node churn in Connect action.
---
 js/id/core/entity.js     | 12 +++++++-----
 test/spec/core/entity.js | 12 +++++++++---
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/js/id/core/entity.js b/js/id/core/entity.js
index b0ed5b0f0..0aaa36907 100644
--- a/js/id/core/entity.js
+++ b/js/id/core/entity.js
@@ -68,17 +68,19 @@ iD.Entity.prototype = {
     },
 
     mergeTags: function(tags) {
-        var merged = _.clone(this.tags);
+        var merged = _.clone(this.tags), changed = false;
         for (var k in tags) {
             var t1 = merged[k],
                 t2 = tags[k];
-            if (t1 && t1 !== t2) {
-                merged[k] = _.union(t1.split(/;\s*/), t2.split(/;\s*/)).join(';');
-            } else {
+            if (!t1) {
+                changed = true;
                 merged[k] = t2;
+            } else if (t1 !== t2) {
+                changed = true;
+                merged[k] = _.union(t1.split(/;\s*/), t2.split(/;\s*/)).join(';');
             }
         }
-        return this.update({tags: merged});
+        return changed ? this.update({tags: merged}) : this;
     },
 
     intersects: function(extent, resolver) {
diff --git a/test/spec/core/entity.js b/test/spec/core/entity.js
index 2cc3cce05..b1ea3d308 100644
--- a/test/spec/core/entity.js
+++ b/test/spec/core/entity.js
@@ -64,9 +64,15 @@ describe('iD.Entity', function () {
     });
 
     describe("#mergeTags", function () {
-        it("returns a new Entity", function () {
-            var a = iD.Entity(),
-                b = a.mergeTags({});
+        it("returns self if unchanged", function () {
+            var a = iD.Entity({tags: {a: 'a'}}),
+                b = a.mergeTags({a: 'a'});
+            expect(a).to.equal(b);
+        });
+
+        it("returns a new Entity if changed", function () {
+            var a = iD.Entity({tags: {a: 'a'}}),
+                b = a.mergeTags({a: 'b'});
             expect(b instanceof iD.Entity).to.be.true;
             expect(a).not.to.equal(b);
         });