Use Session.get() in get_instance instead of Session.query() (#548)

* Use Session.get() in `get_instance` instead of Session.query()

Session.get() tries to fetch an object from the session's identity map if it exists, and performs a SQL query otherwise.

This can help improve load performance by reducing number of SQL queries, especially for nested relationships, if everything is already available into the session.

* Update changelog

---------

Co-authored-by: Steven Loria <sloria1@gmail.com>

AUTHORS.rst

@@ -43,4 +43,5 @@ Contributors
 - Justin Crown `@mrname <https://github.com/mrname>`_
 - Jeppe Fihl-Pearson  `@Tenzer <https://github.com/Tenzer>`_
 - Indivar  `@indiVar0508 <https://github.com/indiVar0508>`_
-- David Doyon  `@ddoyon92 <https://github.com/ddoyon92>`_
+- David Doyon  `@ddoyon92 <https://github.com/ddoyon92>`_
+- Hippolyte Henry `@zippolyte <https://github.com/zippolyte>`_

CHANGELOG.rst

@@ -4,6 +4,11 @@ Changelog
 0.30.0 (unreleased)
 +++++++++++++++++++
 
+Features:
+
+* Use ``Session.get()`` load instances to improve deserialization performance (:pr:`548`).
+  Thanks :user:`zippolyte` for the PR.
+
 Other changes:
 
 * Drop support for Python 3.7, which is EOL (:pr:`540`).

src/marshmallow_sqlalchemy/load_instance_mixin.py

@@ -6,6 +6,7 @@
     Users should not need to use this module directly.
 """
 import marshmallow as ma
+from sqlalchemy.orm.exc import ObjectDeletedError
 
 from .fields import get_primary_keys
 
@@ -55,7 +56,10 @@ def get_instance(self, data):
             props = get_primary_keys(self.opts.model)
             filters = {prop.key: data.get(prop.key) for prop in props}
             if None not in filters.values():
-                return self.session.query(self.opts.model).filter_by(**filters).first()
+                try:
+                    return self.session.get(self.opts.model, filters)
+                except ObjectDeletedError:
+                    return None
             return None
 
         @ma.post_load