overlay/match on array item incorrectly errors (and with wrong expectation)

[jtigger]

This input:

test.yml[1]

  1 ---
  2 clients:
  3 - client1:
  4     secret: blah1
  5 - client2:
  6     secret: blah2
  7
  8 #@ load("@ytt:overlay", "overlay")
  9 #@overlay/match by=overlay.all
 10 ---
 11 clients:
 12 #@overlay/match by=overlay.all,expects="1+"
 13 - client1:
 14     secret: foo

... yields this error message ...

ytt: Error: Overlaying (in following order: test.yml): Document on
line test.yml:10: Map item (key 'clients') on line test.yml:11: Array
item on line test.yml:13: Map item (key 'client1') on line
test.yml:13: Expected number of matched nodes to be 1, but was 0

Instead, I expect this output:

---
clients:
- client1:
    secret: foo
- client1:
    secret: foo

---

[1] https://get-ytt.io/#gist:https://gist.github.com/jtigger/a3dbb1bb167452e288692e32127a4bf5

[cppforlife]

error message is correct. default behaviour is merge, hence it's telling you that second array item doesnt have client1 key already (ie it's not annotated with missing_ok=True if that's indeed what you wanted to do). alternatively to get your expected result you should add overlay/replace to array item:

---
clients:
- client1:
    secret: blah1
- client2:
    secret: blah2

#@ load("@ytt:overlay", "overlay")
#@overlay/match by=overlay.all
---
clients:
#@overlay/match by=overlay.all,expects="1+"
#@overlay/replace
- client1:
    secret: foo

[jtigger]

@cppforlife: when ,expects="1+" is removed, the message reads:

Document on line test.yml:10: Map item (key 'clients') on line
test.yml:11: Array item on line test.yml:13: Expected number of
matched nodes to be 1, but was 2

[cppforlife]

note the difference error messages

Document on line demo.yml:10:
  Map item (key 'clients') on line demo.yml:11:
    Array item on line demo.yml:13:
      Map item (key 'client1') on line demo.yml:14:
        Expected number of matched nodes to be 1, but was 0

vs

Document on line demo.yml:10:
  Map item (key 'clients') on line demo.yml:11:
    Array item on line demo.yml:13:
      Expected number of matched nodes to be 1, but was 2

[cppforlife]

some error message improvements to indicate left vs right nodes

Document on line demo.yml:10 [matched demo.yml:1]
  Map item key 'clients' on line demo.yml:11 [matched demo.yml:2]
    Map item key 'client1' on line demo.yml:12 [did not match]
      Expected number of matched left map items to be 1, but was 0

vs

left=demo.yml right=overlay.yml
left=:1 right=:10 [matched]  Document
left=:2 right=:11 [matched]    Map item key 'clients'
left=?  right=:12 [error]        Map item key 'client1'
                                   Expected number of matched left map items to be 1, but was 0

vs

left=demo.yml:1 right=demo.yml:10 [matched]  Document
left=demo.yml:2 right=demo.yml:11 [matched]    Map item key 'clients'
left=?          right=demo.yml:12 [error]        Map item key 'client1'
                                                   Expected number of matched left map items to be 1, but was 0