Creations happen before deletions in Designs

[nkallergis]

Environment

• Python version: N/A
• Nautobot version: 3.0 but should be the same in 2.x
• nautobot-design-builder version: 3.0.0

Expected Convenient/Desired Behavior

During a Design Deployment run, deletions happen before creations. This way it will be both conceptually more sound but also much more convenient when the object at hand has to be unique (e.g. a Device has to be fully replaced).

Observed Behavior

Creations happen before deletions. (relevant code lines)

Steps to Reproduce

1. Have a Design that creates_or_updates a Device with a specific DeviceType.

devices:
  - "create_or_update:name": "SW01"
    "create_or_update:device_type__model": "C1000"
    role__name: "Core Router"
    status__name: "Planned"
    platform: "!ref:junos_platform"

2. Change the Design to use a different DeviceType.

devices:
  - "create_or_update:name": "SW01"
    "create_or_update:device_type__model": "C2000"
    role__name: "Core Router"
    status__name: "Planned"
    platform: "!ref:junos_platform"

3. You get a validation error due to the Device's uniqueness.

[gsnider2195]

I think this may be a gap in the documentation for create_or_update. I believe that tag should only be used for unique fields on the model, similar to the way you'd use QuerySet.update_or_create. What happens if you do this?

devices:
  - "create_or_update:name": "SW01"
    device_type__model: "C2000"
    role__name: "Core Router"
    status__name: "Planned"
    platform: "!ref:junos_platform"

If we make the requested change to perform deletes first, an incorrectly implemented design could recreate objects every time the design is run instead of updating them. Deleting and then recreating makes it much more difficult to track changes to objects using Nautobot's built-in change logging system because Nautobot uses the primary key to keep track of objects, not the unique fields.

[gsnider2195]

As we discussed today, the implement_design makes changes to the change_set variable. If we flip the create and delete we'll need to figure out how to do a dryrun on the implement_design first so we have the updated change_set.

We'll need to determine if design builder tracks any of the side effects from object creation. For example: if I build a design that creates devices but doesn't declare any interfaces but the device type has interface templates, does design builder have any visibility into those interfaces that are created by the interface templates?

[gsnider2195]

A good point that @smk4664 made: What would happen if you removed DeviceType C1000 in your design? In the current implementation, your device SW01 would be changed to C2000 and then we would delete C1000 and that would work (if SW01 is the only device using C1000). If we reverse the order, we wouldn't be able to delete C1000 first because SW01 is still using it.