Add an age property to DatabaseModel

Most tables have a created_time column, so this will be usable on most
models and is more convenient and readable than always needing to
subtract from utc_now().

tildes/tildes/models/comment/comment.py

@@ -115,7 +115,7 @@ class Comment(DatabaseModel):
             extracted_text, length=200, truncate_at_chars=" "
         )
 
-        if self.created_time and utc_now() - self.created_time > EDIT_GRACE_PERIOD:
+        if self.age > EDIT_GRACE_PERIOD:
             self.last_edited_time = utc_now()
 
     def __repr__(self) -> str:
@@ -197,7 +197,7 @@ class Comment(DatabaseModel):
             acl.append((Allow, "admin", "reply"))
             acl.append((Deny, Everyone, "reply"))
 
-        if utc_now() - self.created_time < timedelta(hours=2):
+        if self.age < timedelta(hours=2):
             acl.append((Deny, "comment.reply_slow", "reply"))
 
         acl.append((Allow, Authenticated, "reply"))

tildes/tildes/models/database_model.py

@@ -3,6 +3,7 @@
 
 """Contains the base DatabaseModel class."""
 
+from datetime import timedelta
 from typing import Any, Optional, Type, TypeVar
 
 from marshmallow import Schema
@@ -11,6 +12,8 @@ from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.schema import MetaData
 from sqlalchemy.sql.schema import Table
 
+from tildes.lib.datetime import utc_now
+
 
 ModelType = TypeVar("ModelType")  # pylint: disable=invalid-name
 
@@ -82,6 +85,18 @@ class DatabaseModelBase:
 
         return self._schema
 
+    @property
+    def age(self) -> timedelta:
+        """Return the model's age - requires it to have a `created_time` column."""
+        if not hasattr(self, "created_time"):
+            raise AttributeError("'age' attribute requires 'created_time' column.")
+
+        # created_time should only be None during __init__, age of 0 is reasonable
+        if self.created_time is None:  # type: ignore
+            return timedelta(0)
+
+        return utc_now() - self.created_time  # type: ignore
+
     def _validate_new_value(self, attribute: str, value: Any) -> Any:
         """Validate the new value for a column.
 

tildes/tildes/models/topic/topic.py

@@ -158,7 +158,7 @@ class Topic(DatabaseModel):
         self._markdown = new_markdown
         self.rendered_html = convert_markdown_to_safe_html(new_markdown)
 
-        if self.created_time and utc_now() - self.created_time > EDIT_GRACE_PERIOD:
+        if self.age > EDIT_GRACE_PERIOD:
             self.last_edited_time = utc_now()
 
     @hybrid_property

tildes/tildes/models/user/user.py

@@ -309,7 +309,7 @@ class User(DatabaseModel):
             raise ValueError("Unknown permissions format")
 
         # give the user the "comment.label" permission if they're over a week old
-        if utc_now() - self.created_time > timedelta(days=7):
+        if self.age > timedelta(days=7):
             principals.append("comment.label")
 
         return principals