Add conflict resolution API

New methods:
  conflict.resolveUsingSource(cb)
  conflict.resolveUsingTarget(cb)
  conflict.resolveManually(data, cb)

common/models/change.js

@@ -616,6 +616,13 @@ module.exports = function(Change) {
   /**
    * Resolve the conflict.
    *
+   * Set the source change's previous revision to the current revision of the
+   * (conflicting) target change. Since the changes are no longer conflicting
+   * and appear as if the source change was based on the target, they will be
+   * replicated normally as part of the next replicate() call.
+   *
+   * This is effectively resolving the conflict using the source version.
+   *
    * @callback {Function} callback
    * @param {Error} err
    */
@@ -629,6 +636,74 @@ module.exports = function(Change) {
     });
   };
 
+  /**
+   * Resolve the conflict using the instance data in the source model.
+   *
+   * @callback {Function} callback
+   * @param {Error} err
+   */
+  Conflict.prototype.resolveUsingSource = function(cb) {
+    this.resolve(function(err) {
+      // don't forward any cb arguments from resolve()
+      cb(err);
+    });
+  };
+
+  /**
+   * Resolve the conflict using the instance data in the target model.
+   *
+   * @callback {Function} callback
+   * @param {Error} err
+   */
+  Conflict.prototype.resolveUsingTarget = function(cb) {
+    var conflict = this;
+
+    conflict.models(function(err, source, target) {
+      if (err) return done(err);
+      if (target === null) {
+        return conflict.SourceModel.deleteById(conflict.modelId, done);
+      }
+      var inst = new conflict.SourceModel(target);
+      inst.save(done);
+    });
+
+    function done(err) {
+      // don't forward any cb arguments from internal calls
+      cb(err);
+    }
+  };
+
+  /**
+   * Resolve the conflict using the supplied instance data.
+   *
+   * @param {Object} data The set of changes to apply on the model
+   * instance. Use `null` value to delete the source instance instead.
+   * @callback {Function} callback
+   * @param {Error} err
+   */
+
+  Conflict.prototype.resolveManually = function(data, cb) {
+    var conflict = this;
+    if (!data) {
+      return conflict.SourceModel.deleteById(conflict.modelId, done);
+    }
+
+    conflict.models(function(err, source, target) {
+      if (err) return done(err);
+      var inst = source || new conflict.SourceModel(target);
+      inst.setAttributes(data);
+      inst.save(function(err) {
+        if (err) return done(err);
+        conflict.resolve(done);
+      });
+    });
+
+    function done(err) {
+      // don't forward any cb arguments from internal calls
+      cb(err);
+    }
+  };
+
   /**
    * Determine the conflict type.
    *

test/replication.test.js

@@ -963,50 +963,70 @@ describe('Replication / Change APIs', function() {
           ], done);
         });
 
-        it('handles conflict resolved using "ours"', function(done) {
-          testResolvedConflictIsHandledWithNoMoreConflicts(
+        it('handles UPDATE conflict resolved using "ours"', function(done) {
+          testUpdateConflictIsResolved(
             function resolveUsingOurs(conflict, cb) {
-              conflict.resolve(cb);
+              conflict.resolveUsingSource(cb);
             },
             done);
         });
 
-        it('handles conflict resolved using "theirs"', function(done) {
-          testResolvedConflictIsHandledWithNoMoreConflicts(
+        it('handles UPDATE conflict resolved using "theirs"', function(done) {
+          testUpdateConflictIsResolved(
             function resolveUsingTheirs(conflict, cb) {
-              conflict.models(function(err, source, target) {
-                if (err) return cb(err);
               // We sync ClientA->Server first
               expect(conflict.SourceModel.modelName)
                 .to.equal(ClientB.modelName);
-                var m = new conflict.SourceModel(target);
-                m.save(cb);
-              });
+              conflict.resolveUsingTarget(cb);
             },
             done);
         });
 
-        it('handles conflict resolved manually', function(done) {
-          testResolvedConflictIsHandledWithNoMoreConflicts(
+        it('handles UPDATE conflict resolved manually', function(done) {
+          testUpdateConflictIsResolved(
             function resolveManually(conflict, cb) {
-              conflict.models(function(err, source, target) {
-                if (err) return cb(err);
-                var m = source || new conflict.SourceModel(target);
-                m.name = 'manual';
-                m.save(function(err) {
-                  if (err) return cb(err);
-                  conflict.resolve(function(err) {
-                    if (err) return cb(err);
-                    cb();
+              conflict.resolveManually({ name: 'manual' }, cb);
+            },
+            done);
         });
+
+        it('handles DELETE conflict resolved using "ours"', function(done) {
+          testDeleteConflictIsResolved(
+            function resolveUsingOurs(conflict, cb) {
+              conflict.resolveUsingSource(cb);
+            },
+            done);
         });
+
+        it('handles DELETE conflict resolved using "theirs"', function(done) {
+          testDeleteConflictIsResolved(
+            function resolveUsingTheirs(conflict, cb) {
+              // We sync ClientA->Server first
+              expect(conflict.SourceModel.modelName)
+                .to.equal(ClientB.modelName);
+              conflict.resolveUsingTarget(cb);
+            },
+            done);
         });
+
+        it('handles DELETE conflict resolved as manual delete', function(done) {
+          testDeleteConflictIsResolved(
+            function resolveManually(conflict, cb) {
+              conflict.resolveManually(null, cb);
+            },
+            done);
+        });
+
+        it('handles DELETE conflict resolved manually', function(done) {
+          testDeleteConflictIsResolved(
+            function resolveManually(conflict, cb) {
+              conflict.resolveManually({ name: 'manual' }, cb);
             },
             done);
         });
       });
 
-      function testResolvedConflictIsHandledWithNoMoreConflicts(resolver, cb) {
+      function testUpdateConflictIsResolved(resolver, cb) {
         async.series([
           // sync the new model to ClientB
           sync(ClientB, Server),
@@ -1041,6 +1061,44 @@ describe('Replication / Change APIs', function() {
         ], cb);
       }
 
+      function testDeleteConflictIsResolved(resolver, cb) {
+        async.series([
+          // sync the new model to ClientB
+          sync(ClientB, Server),
+          verifyInstanceWasReplicated(ClientA, ClientB, sourceInstanceId),
+
+          // ClientA makes a change
+          function deleteInstanceOnClientA(next) {
+            ClientA.deleteById(sourceInstanceId, next);
+          },
+
+          sync(ClientA, Server),
+
+          // ClientB changes the same instance
+          updateClientB('b'),
+
+          function syncAndResolveConflict(next) {
+            replicate(ClientB, Server, function(err, conflicts, cps) {
+              if (err) return next(err);
+
+              expect(conflicts).to.have.length(1);
+              expect(conflicts[0].SourceModel.modelName)
+                .to.equal(ClientB.modelName);
+
+              debug('Resolving the conflict %j', conflicts[0]);
+              resolver(conflicts[0], next);
+            });
+          },
+
+          // repeat the last sync, it should pass now
+          sync(ClientB, Server),
+          // and sync back to ClientA too
+          sync(ClientA, Server),
+
+          verifyInstanceWasReplicated(ClientB, ClientA, sourceInstanceId)
+        ], cb);
+      }
+
       function updateClientB(name) {
         return function updateInstanceB(next) {
           ClientB.findById(sourceInstanceId, function(err, instance) {