Use SQLAlchemy object lifecycle event for metrics

The creation of the Theme Previews page showed that the way I was doing
metrics for database model objects being created wasn't very good.
Whenever someone loaded the Theme Previews page, the "total topics"
metric would increase by 2, "total comments" by 4, and "exemplary
labels" by 1, because the page is creating that many fake objects and
the metrics were being sent in each class's __init__ method.

This changes to take advantage of SQLAlchemy's object lifecycle event
for "pending to persistent", which only triggers when an object is
actually persisted to the database. When this event happens, the
object's _update_creation_metric method is called, and all metric
updates have been moved into that method now.

tildes/tildes/database.py

@@ -7,7 +7,7 @@ from typing import Callable, Type
 
 from pyramid.config import Configurator
 from pyramid.request import Request
-from sqlalchemy import engine_from_config
+from sqlalchemy import engine_from_config, event
 from sqlalchemy.orm import sessionmaker
 from sqlalchemy.orm.session import Session
 from sqlalchemy.pool import NullPool
@@ -92,6 +92,15 @@ def includeme(config: Configurator) -> None:
         reify=True,
     )
 
+    # add a listener to the session to update database model creation metrics when
+    # any object goes through the "pending to persistent" state change
+    event.listen(
+        session_factory,
+        "pending_to_persistent",
+        # pylint: disable=protected-access
+        lambda session, instance: instance._update_creation_metric(),
+    )
+
     config.add_request_method(query_factory, "query")
 
     config.add_request_method(obtain_lock, "obtain_lock")

tildes/tildes/models/comment/comment.py

@@ -141,6 +141,7 @@ class Comment(DatabaseModel):
         self.markdown = markdown
         self.parent_comment = parent_comment
 
+    def _update_creation_metric(self) -> None:
         incr_counter("comments")
 
     def __acl__(self) -> Sequence[Tuple[str, Any, str]]:

tildes/tildes/models/comment/comment_label.py

@@ -64,7 +64,8 @@ class CommentLabel(DatabaseModel):
         self.weight = weight
         self.reason = reason
 
-        incr_counter("comment_labels", label=label.name)
+    def _update_creation_metric(self) -> None:
+        incr_counter("comment_labels", label=self.label.name)
 
     @property
     def name(self) -> str:

tildes/tildes/models/comment/comment_vote.py

@@ -48,4 +48,5 @@ class CommentVote(DatabaseModel):
         self.user = user
         self.comment = comment
 
+    def _update_creation_metric(self) -> None:
         incr_counter("votes", target_type="comment")

tildes/tildes/models/database_model.py

@@ -97,6 +97,18 @@ class DatabaseModelBase:
 
         return utc_now() - self.created_time  # type: ignore
 
+    def _update_creation_metric(self) -> None:
+        """Update the metric tracking creations of this model type.
+
+        This function will be attached to the SQLAlchemy Object Lifecycle event for the
+        "pending to persistent" transition, which occurs when an object is persisted to
+        the database. This ensures that the metric is only updated when an object is
+        truly created in the database, not just whenever the model class is initialized.
+
+        Model classes that have a creation metric should override this method.
+        """
+        pass
+
     def _validate_new_value(self, attribute: str, value: Any) -> Any:
         """Validate the new value for a column.
 

tildes/tildes/models/group/group_subscription.py

@@ -48,4 +48,5 @@ class GroupSubscription(DatabaseModel):
         self.user = user
         self.group = group
 
+    def _update_creation_metric(self) -> None:
         incr_counter("subscriptions")

tildes/tildes/models/message/message.py

@@ -118,6 +118,7 @@ class MessageConversation(DatabaseModel):
         self.markdown = markdown
         self.rendered_html = convert_markdown_to_safe_html(markdown)
 
+    def _update_creation_metric(self) -> None:
         incr_counter("messages", type="conversation")
 
     def __acl__(self) -> Sequence[Tuple[str, Any, str]]:
@@ -243,6 +244,7 @@ class MessageReply(DatabaseModel):
         self.markdown = markdown
         self.rendered_html = convert_markdown_to_safe_html(markdown)
 
+    def _update_creation_metric(self) -> None:
         incr_counter("messages", type="reply")
 
     @property

tildes/tildes/models/topic/topic.py

@@ -217,8 +217,6 @@ class Topic(DatabaseModel):
         new_topic.topic_type = TopicType.TEXT
         new_topic.markdown = markdown
 
-        incr_counter("topics", type="text")
-
         return new_topic
 
     @classmethod
@@ -230,10 +228,11 @@ class Topic(DatabaseModel):
         new_topic.topic_type = TopicType.LINK
         new_topic.link = link
 
-        incr_counter("topics", type="link")
-
         return new_topic
 
+    def _update_creation_metric(self) -> None:
+        incr_counter("topics", type=self.topic_type.name.lower())
+
     def __acl__(self) -> Sequence[Tuple[str, Any, str]]:
         """Pyramid security ACL."""
         acl = []

tildes/tildes/models/topic/topic_vote.py

@@ -48,4 +48,5 @@ class TopicVote(DatabaseModel):
         self.user = user
         self.topic = topic
 
+    def _update_creation_metric(self) -> None:
         incr_counter("votes", target_type="topic")