Document & test relationship functionality
Create unit tests for relationship handling:
- Consistent behaviour across different relationship types, including polymorphs
- Pivot table values handled appropriately
- sync strategy vs its inverse
- Creating related records
- Deleting related records
- Filtering the parent models on values in related records
- Ensure documentation in README.md for relationships is separated into docs/Relationships.md and is accurate for the desired relationship behaviour, including examples.
Note this issue is not to change any existing behaviour, but to test if the existing behaviour meets the expected / documented behaviours. We'll create follow-up issues for any failing tests, and plan any required changes in behaviour, as they could have wide effects on existing projects.
10/2/2
This follows on to the following issues (which may crop up during this process - and should be addressed inteh following order)