diff --git a/doc/dev/debug.md b/doc/dev/debug.md
new file mode 100644
index 00000000..f6cb73e6
--- /dev/null
+++ b/doc/dev/debug.md
@@ -0,0 +1,6 @@
+# Debugging
+
+## Show SQL commands
+
+If you set the `YOKADI_SQL_DEBUG` environment variable to a value different
+from "0", all SQL commands will be printed to stdout.
diff --git a/yokadi/core/db.py b/yokadi/core/db.py
index a72aa7a8..da7d324a 100644
--- a/yokadi/core/db.py
+++ b/yokadi/core/db.py
@@ -64,8 +64,12 @@ def merge(self, session, other):
         """Merge other into us"""
         if self is other:
             raise YokadiException("Cannot merge a project into itself")
-        for task in other.tasks:
-            task.project = self
+        # Do a bulk-update: it's faster than updating each task individually
+        session.query(Task).filter_by(projectId=other.id).update({Task.projectId: self.id})
+
+        # Tell SQLAlchemy to forget everything it knows about `other`,
+        # otherwise it will delete the tasks which were once attached to it
+        session.expire(other)
         session.delete(other)
 
 
@@ -289,7 +293,8 @@ def __init__(self, dbFileName, createIfNeeded=True, memoryDatabase=False, update
         if memoryDatabase:
             connectionString = "sqlite:///:memory:"
 
-        self.engine = create_engine(connectionString)
+        echo = os.environ.get("YOKADI_SQL_DEBUG", "0") != "0"
+        self.engine = create_engine(connectionString, echo=echo)
         Session = sessionmaker(bind=self.engine)
         self.session = Session()
 
diff --git a/yokadi/tests/projecttestcase.py b/yokadi/tests/projecttestcase.py
index 1dd9fce0..e79f5134 100644
--- a/yokadi/tests/projecttestcase.py
+++ b/yokadi/tests/projecttestcase.py
@@ -78,20 +78,24 @@ def testStatus(self):
         self.assertEqual(project.active, True)
 
     def testMerge(self):
-        # Create project p1 with one associated task
-        dbutils.addTask("p1", "t1", interactive=False)
-
-        # Create project p2 with one associated task
-        dbutils.addTask("p2", "t2", interactive=False)
+        COUNT = 4
+        for x in range(COUNT):
+            dbutils.addTask('p1', 'p1-t{}'.format(x), interactive=False)
+            dbutils.addTask('p2', 'p2-t{}'.format(x), interactive=False)
 
         # Merge p1 into p2
         tui.addInputAnswers("y")
         self.cmd.do_p_merge("p1 p2")
 
-        # p2 should have two tasks now
+        # p2 should have both its tasks and all p1 tasks now
         project = self.session.query(Project).filter_by(name="p2").one()
         tasks = set([x.title for x in project.tasks])
-        self.assertEquals(tasks, {"t1", "t2"})
+
+        expected = set()
+        for x in range(COUNT):
+            expected.add('p1-t{}'.format(x))
+            expected.add('p2-t{}'.format(x))
+        self.assertEqual(tasks, expected)
 
         # p1 should be gone
         testutils.assertQueryEmpty(self, self.session.query(Project).filter_by(name="p1"))
diff --git a/yokadi/ycli/projectcmd.py b/yokadi/ycli/projectcmd.py
index 872ef343..2863958f 100644
--- a/yokadi/ycli/projectcmd.py
+++ b/yokadi/ycli/projectcmd.py
@@ -139,7 +139,6 @@ def parser_p_merge(self):
         return parser
 
     def do_p_merge(self, line):
-        raise Exception("Broken for now, going to fix it")
         session = db.getSession()
         parser = self.parser_p_merge()
         args = parser.parse_args(line)