Ansible yaml anchors and jinja2 templating - ansible

Question

how do I overwrite the disk attribute while also not hardcoding the number of disks?
This is what I want it (the tasks/main.yml of that role) to do, but it fails with an syntax error and also requires hardcoding the number of disks:
---
- name: anchors
when: false
debug:
new_disk:
- &new_disk
size_gb: 80
type: thin
datastore: '{{ item.datastore }}'
- name: Deploy usage001 vms
loop: '{{ vms.usage001 }}
vmware_guest:
disk:
- <<: *new_disk
- <<: *new_disk
'{{ item.disk[0] }}'
- <<: *new_disk
'{{ item.disk[1] }}
Where item looks like:
vms:
usage001:
disk:
- size_gb: 1000
- size_gb: 600
usage002:
(...)

Answer 1

The documentation for <<, the Merge Key Language Independent Type states:
The “<<” merge key is used to indicate that all the keys of one or more specified maps should be inserted into the current map.
But you specify the anchor new-disk on a sequence instead of map.
You probably want to do:
new_disk:
- &new_disk
size_gb: 80
type: thin
datastore: '{{ item.datastore }}'
You seem to want select size_gb: 1000 from your item, but as the quotes are outside of your jinja2 syntax the substitution, if it works, will result in:
- <<: *new_disk
'size_gb: 1000'
and for that to work it has to be:
- <<: *new_disk
size_gb: 1000
so make sure you get rid of those quotes.
The selection using item.disk[0] given your item seems strange as well, I would have expected something like item.vms.usage001.disk[0] but that might be my lack of jinja2 specific knowledge.