How to add an object to a list if it doesn't exist?

[notorand-it]

I am not sure I can really do this in with a single command.
Of course I know how to do it with 2 (query + conditional add).
This is the file:

list:
  - name: one
    value: 1
  - name: two
    value: 2

I would like to add/replace [{"name": "one", "value": "1"}] only if the object with name "one" is not in the list, so only a single object is present in the list with the propery "name": "one".
Any hint?

[jerrybean]

hi,i would like to know how to do this in multi commands?

[notorand-it]

You first query and in case it is not there you add.

In bash it can be something like yq -e ... || yq ... where the 1st one queries and the 2nd one appends.

[mikefarah]

You can do it like this:

yq 'with(.list; select(all_c(.name != "one")) | . += {"name": "on"} )' file.yaml

Explanation:

• High level: we want to select 'list' only if it doesn't already have an entry with name: one. Then well add to that. Note that if we don't match anything with select, then the append won't do anything.
• with(.list; )` evaluates with .list as the root. Its just syntactic sugar so you don't have to do .list everywhere in
• Now we match the list when there are no name: one - that is, all the entries in the list do not have that: select(all_c(.name != "one")).
• Next, we append an object to that list . += {"name": "on"}

[mikefarah]

See https://mikefarah.gitbook.io/yq/operators/boolean-operators#all_c-returns-true-if-all-elements-in-the-array-are-true-for-the-given-condition; https://mikefarah.gitbook.io/yq/operators/select and https://mikefarah.gitbook.io/yq/operators/with

[notorand-it]

Cool! Thanks

[sel]

Thanks - really useful!

In the case where I needed this list may or may not exist, so I learnt about the Alternative (default value) operator and used it as follows:

yq 'with(.list // (.list=[]); select(all_c(.name != "one")) | . += {"name": "on"} )' file.yaml

I probably use yq every day - thanks for creating such a great tool!

[audunsol]

I know this is answered very elegantly, but I just came here and realized I had a different solution that may help someone (for me the following was simpler to reason about):

yq '([{"name": "one", "value": "1"}] + .list | unique_by(.name)) as $uniqueList | .list = $uniqueList' file.yaml

Explanation:

• add the item to beginning of the existing list (don't care about duplication)
• use unique_by(.name) to filter out the duplicates
• store the unique list in a temporary variable $uniqueList
• finally, assign the new unique list to .list

Note that this would remove existing duplicates as well, if you already have them in the original list (which you may or may not want).