diff --git a/swh/model/hypothesis_strategies.py b/swh/model/hypothesis_strategies.py
--- a/swh/model/hypothesis_strategies.py
+++ b/swh/model/hypothesis_strategies.py
@@ -45,7 +45,7 @@
     return builds(
         TimestampWithTimezone,
         timestamp=timestamps(),
-        offset=integers(-2**16, 2**16-1))
+        offset=integers(min_value=-14*60, max_value=14*60))
 
 
 def origins():
@@ -77,8 +77,8 @@
         id=sha1_git(),
         date=timestamps_with_timezone(),
         committer_date=timestamps_with_timezone(),
-        parents=lists(binary()),
-        directory=binary(),
+        parents=lists(sha1_git()),
+        directory=sha1_git(),
         metadata=one_of(none(), dictionaries(binary(), binary())))
 
 
diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -48,6 +48,13 @@
     def to_dict(self):
         return attr.asdict(self)
 
+    @offset.validator
+    def check_offset(self, attribute, value):
+        if not (-2**15 <= value < 2**15):
+            # max 14 hours offset in theory, but you never know what
+            # you'll find in the wild...
+            raise ValueError('offset too big: %d minutes' % value)
+
 
 @attr.s
 class Origin:
@@ -134,6 +141,14 @@
         return rel
 
 
+class RevisionType(Enum):
+    GIT = 'git'
+    TAR = 'tar'
+    DSC = 'dsc'
+    SUBVERSION = 'svn'
+    MERCURIAL = 'hg'
+
+
 @attr.s
 class Revision:
     id = attr.ib(type=Sha1Git)
@@ -143,7 +158,7 @@
     date = attr.ib(type=TimestampWithTimezone)
     committer_date = attr.ib(type=TimestampWithTimezone)
     parents = attr.ib(type=List[Sha1Git])
-    type = attr.ib(type=str)
+    type = attr.ib(type=RevisionType)
     directory = attr.ib(type=Sha1Git)
     metadata = attr.ib(type=Optional[dict])
     synthetic = attr.ib(type=bool)
@@ -152,6 +167,7 @@
         rev = attr.asdict(self)
         rev['date'] = self.date.to_dict()
         rev['committer_date'] = self.committer_date.to_dict()
+        rev['type'] = rev['type'].value
         return rev