jazzband#1266 resolving through tables defined as strings on m2m rela…

…tions

simple_history/models.py

@@ -15,7 +15,7 @@
 from django.db import models
 from django.db.models import ManyToManyField
 from django.db.models.fields.proxy import OrderWrt
-from django.db.models.fields.related import ForeignKey
+from django.db.models.fields.related import ForeignKey, lazy_related_operation
 from django.db.models.fields.related_descriptors import (
     ForwardManyToOneDescriptor,
     ReverseManyToOneDescriptor,
@@ -84,6 +84,8 @@ class HistoricalRecords:
     DEFAULT_MODEL_NAME_PREFIX = "Historical"
 
     thread = context = LocalContext()  # retain thread for backwards compatibility
+    # Key is the m2m field and value is a tuple where first entry is the historical m2m
+    # model and second is the through model
     m2m_models = {}
 
     def __init__(
@@ -238,15 +240,24 @@ def finalize(self, sender, **kwargs):
         sender._meta.simple_history_manager_attribute = self.manager_name
 
         for field in m2m_fields:
-            m2m_model = self.create_history_m2m_model(
-                history_model, field.remote_field.through
-            )
-            self.m2m_models[field] = m2m_model
 
-            setattr(module, m2m_model.__name__, m2m_model)
+            def resolve_through_model(history_model, through_model):
+                m2m_model = self.create_history_m2m_model(history_model, through_model)
+                # Save the created history model and the resolved through model together
+                # for reference later
+                self.m2m_models[field] = (m2m_model, through_model)
+
+                setattr(module, m2m_model.__name__, m2m_model)
 
-            m2m_descriptor = HistoryDescriptor(m2m_model)
-            setattr(history_model, field.name, m2m_descriptor)
+                m2m_descriptor = HistoryDescriptor(m2m_model)
+                setattr(history_model, field.name, m2m_descriptor)
+
+            # Lazily generate the historical m2m models for the fields when all of the
+            # associated models have been fully loaded. This handles resolving through
+            # models referenced as strings. This is how django m2m fields handle this.
+            lazy_related_operation(
+                resolve_through_model, history_model, field.remote_field.through
+            )
 
     def get_history_model_name(self, model):
         if not self.custom_model_name:
@@ -685,9 +696,7 @@ def m2m_changed(self, instance, action, attr, pk_set, reverse, **_):
 
     def create_historical_record_m2ms(self, history_instance, instance):
         for field in history_instance._history_m2m_fields:
-            m2m_history_model = self.m2m_models[field]
-            original_instance = history_instance.instance
-            through_model = getattr(original_instance, field.name).through
+            m2m_history_model, through_model = self.m2m_models[field]
             through_model_field_names = [f.name for f in through_model._meta.fields]
             through_model_fk_field_names = [
                 f.name for f in through_model._meta.fields if isinstance(f, ForeignKey)