Possible solution for admin inlines

[tomH01]

My company uses simple history as well. I found a solution to display the correct inlines in the admin (sketched below).
If you are interested in the solution, I would obtain approval from my company to contribute the solution.
However, the solutions has some workarounds that would be nice to be replaced firstly:

• Always Save Inlines: All inlines have to be saved whenever the parent model is saved (solved by AlwaysSaveForm in the inline class). It would be better to handle this directly within django-simple-history.
• Save Order: The standard Django save order has to be used (Parent first, Children afterward)

Solution Overview

Prerequisites:

• Inlines are always within a ForeignKey relationship (Child points to Parent)
• HistoricalModels deliver a version field history_id
• When a Parent is changed a new historical parent record with a history_id is created
• When modifying the inlines of a Parent, only the affected Children will have a new historical record

Implementation:

1. Always Save Inlines: Use an AlwaysSaveForm to ensure all Children are saved whenever the Parent is saved.
2. Mapping: For each foreign key of a Child add a new field in the historical child model to create a mapping to the Parent history_id:

• Distiguish by history_type of Child: "No change", "Created (+)", "Deleted (-)", "Modified (~)"
• If "No change" or "Created (+)" or "Modified (~)" then add latest Parent history_id
• Else change nothing
  
• Display the Inlines: Display the inlines with parts of Add support for admin inlines #315 while filtering them by their mapping field.