Circuit termination connected endpoint does not show up in API response

[ggidofalvy-tc]

Environment

• Python version: 3.7.3
• NetBox version: 2.9.3

Steps to Reproduce

1. Create circuit
2. Create A-side termination
3. Connect A-side termination to arbitrary interface on a device
4. Create Z-side termination
5. Connect Z-side termination to arbitrary interface on a device
6. Check web interface + API result of circuit

Expected Behavior

The web interface and the API shows the connected interfaces/devices on both A and Z sides.

Observed Behavior

The web interface shows the connected interfaces/devices on both A and Z sides; however, the API only shows the A side as connected.

Deleting the terminations and creating the Z-side termination first, then the A-side termination leads to the A-side termination not showing any connected endpoints when queried through the API.

Swapping terminations leads to the API showing the A-side as having nothing connected to it.

The actual circuit terminations created are also affected by this issue.

The 'fix' for this issue did not fix the problem with the model itself, it is a mere workaround, querying the attached cable (and not the connected_endpoint, which can be null).

[ggidofalvy-tc]

Seems to be a duplicate of #4851 ?

Also only affects new circuits. The issue began after updating from v2.7.10.

[ggidofalvy-tc]

For "end-user" developers who ran into this issue: A workaround is to query the cable within the circuit termination, then use the termination within the cable that isn't the original circuit termination (this is what the previous fix essentially did).

[ggidofalvy-tc]

Doing another reproduction pass revealed that there are some cases when both circuit terminations show their connected_endpoint as null, even when both are connected.

[jeremystretch]

This appears to be the same issue described in #4925.