multiply (merge) operator on map with multiple files results in empty map

[timoreimann]

Hello 👋

Describe the bug
I am trying to merge a YAML file containing a map (dictionary) into another YAML file using the *? operator (merge on existing keys). Instead of merging in the map items, however, an empty map is produced.

Version of yq: 4.9.8
Operating system: Mac OS X (Big Sur)
Installed via: brew

Input Yaml
1.yaml:

thing: one
cat: frog

2.yaml:

thing:
  field1: one
  field2: two

Command

yq eval-all 'select(fileIndex == 0) *? select(fileIndex == 1)' 1.yaml 2.yaml

Actual behavior

thing: {}
cat: frog

Expected behavior

thing:
  field1: one
  field2: two
cat: frog

Additional context
The * works fine per se, but of course ignores the existing key constraint which is what I need for my specific case. (Omitted from my examples above to keep the reproduction as simple as possible.)

[mikefarah]

Yeah I see, this is a bit tricky - merge runs recursively, and the ? flag means it shouldn't create new keys. As it recurses, it will first encounter thing it will switch the type from a scalar to a map, making it empty. Next it will encounter field1 and try to update that in thing - however, thing is empty and does not have that field, so it skips over it.

Have to think about this one a little more

[ybizeul]

I think this exposes the same behavior :

Input File:

a:
 image: image1
b:
 image: image2
c:
 key: value
---

---

Command:

❯ cat test.yaml|yq e 'select(.a.image).a.image = "test/"+.a.image'
a:
  image: test/image1
b:
  image: image2
c:
  key: value
---
{}
---
{}