On restore bypass assignment for snapshot object data where the associated column no longer exists (#66)

Ensure `SnapshotItem#restore_item!` and `Snapshot#fetch_reified_items` bypass assignment for snapshot object data where the associated column no longer exists.

Solves #58

Author: westonganger

CHANGELOG.md

@@ -3,6 +3,7 @@ CHANGELOG
 
 - **Unreleased**
   * [View Diff](https://github.com/westonganger/active_snapshot/compare/v0.5.0...master)
+  * [#66](https://github.com/westonganger/active_snapshot/pull/66) - Ensure `SnapshotItem#restore_item!` and `Snapshot#fetch_reified_items` bypass assignment for snapshot object data where the associated column no longer exists.
   * [#63](https://github.com/westonganger/active_snapshot/pull/63) - Fix bug when enum value is nil
 
 - **v0.5.0** - Nov 8, 2024

README.md

@@ -131,9 +131,11 @@ end
 
 Now when you run `create_snapshot!` the associations will be tracked accordingly
 
-# Reifying Snapshot Items
+# Reifying Snapshots
 
-You can view all of the reified snapshot items by calling the following method. Its completely up to you on how to use this data.
+A reified record refers to an ActiveRecord instance where the local objects data is set to match the snaphotted data, but the database remains changed.
+
+You can view all of the "reified" snapshot items by calling the following method. Its completely up to you on how to use this data.
 
 ```ruby
 reified_parent, reified_children_hash = snapshot.fetch_reified_items
@@ -166,6 +168,32 @@ attrs_not_changed = old_attrs.to_a.intersection(new_attrs.to_a).to_h
 attrs_changed = new_attrs.to_a - attrs_not_changed.to_a
 ```
 
+# Important Data Considerations / Warnings
+
+### Dropping columns
+
+If you plan to use the snapshot restore capabilities please be aware:
+
+Whenever you drop a database column and there already exists snapshots of that model then you are kind of silently breaking your restore mechanism. Because now the application will not be able to assign data to columns that dont exist on the model. We work around this by bypassing the attribute assignment for snapshot item object entries that does not correlate to a current database column.
+
+I recommend that you add an entry to this in your applications safe-migrations guidelines.
+
+If you would like to detect if this situation has already ocurred you can use the following script:
+
+```ruby
+SnapshotItem.all.each do |snapshot_item|
+  snapshot_item.object.keys.each do |key|
+    klass = Class.const_get(snapshot_item.item_type)
+
+    if !klass.column_names.include?(key)
+      invalid_data = snapshot_item.object.slice(*klass.column_names)
+
+      raise "invalid data found - #{invalid_data}"
+    end
+  end
+end
+```
+
 # Key Models Provided & Additional Customizations
 
 A key aspect of this library is its simplicity and small API. For major functionality customizations we encourage you to first delete this gem and then copy this gems code directly into your repository.

lib/active_snapshot/models/snapshot.rb

@@ -112,7 +112,15 @@ def fetch_reified_items(readonly: true)
       reified_parent = nil
 
       snapshot_items.each do |si|
-        reified_item = si.item_type.constantize.new(si.object)
+        reified_item = si.item_type.constantize.new
+
+        si.object.each do |k,v|
+          if reified_item.respond_to?("#{k}=")
+            reified_item[k] = v
+          else
+            # database column was likely dropped since the snapshot was created
+          end
+        end
 
         if readonly
           reified_item.readonly!

lib/active_snapshot/models/snapshot_item.rb

@@ -51,7 +51,13 @@ def restore_item!
         self.item = item_klass.new
       end
 
-      item.assign_attributes(object)
+      object.each do |k,v|
+        if item.respond_to?("#{k}=")
+          item[k] = v
+        else
+          # database column was likely dropped since the snapshot was created
+        end
+      end
 
       item.save!(validate: false, touch: false)
     end

test/models/snapshot_item_test.rb

@@ -67,4 +67,17 @@ def test_restore_item!
     @snapshot_item.restore_item!
   end
 
+  def test_restore_item_handles_dropped_columns!
+    snapshot = @snapshot_klass.includes(:snapshot_items).first
+
+    snapshot_item = snapshot.snapshot_items.first
+
+    attrs = snapshot_item.object
+    attrs["foo"] = "bar"
+
+    snapshot_item.update!(object: attrs)
+
+    snapshot_item.restore_item!
+  end
+
 end

test/models/snapshot_test.rb

@@ -192,6 +192,19 @@ def test_fetch_reified_items_with_sti_class
     assert_equal comment_content, reified_items.second[:comments].first.content
   end
 
+  def test_fetch_reified_items_handles_dropped_columns!
+    snapshot = @snapshot_klass.first
+
+    snapshot_item = snapshot.snapshot_items.first
+
+    attrs = snapshot_item.object
+    attrs["foo"] = "bar"
+
+    snapshot_item.update!(object: attrs)
+
+    reified_items = snapshot.fetch_reified_items(readonly: false)
+  end
+
   def test_single_model_snapshots_without_children
     instance = ParentWithoutChildren.create!({a: 1, b: 2})