JQ: How to join arrays by key? - jq

Question

How can I expand array with a category stored in a dictionary?
E.g. for the following input,
{"weigths":
[
{"name":"apple","weigth":200},
{"name":"tomato", "weigth":100}
],
"categories":
[
{"name":"apple","category":"fruit"},
{"name":"tomato","category":"vegetable"}
]
}
I need a performance-efficient way to append respective category to each object in weights, resulting in output:
{"weigths":
[
{"name":"apple","weigth":200, "category": "fruit"},
{"name":"tomato", "weigth":100, "category": "vegetable"}
],
}
Is it something for JOIN/4? (I never tried it)
Update:
Ideally I would like to deal with a not-that-SQL object for categories: full input looking like this
{"weigths":
[
{"name":"apple","weigth":200},
{"name":"orange", "weigth":300}
{"name":"tomato","weigth":100},
{"name":"spinach","weigth":50},
],
"categories":
{
"fruit": ["apple", "orange"],
"vegetable": ["tomato", "spinach"]
}
}
...still getting a similar output:
{"weigths":
[
{"name": "apple", "weigth": 200, "category": "fruit"},
{"name": "orange", "weigth": 300, "category": "fruit"},
{"name": "tomato", "weigth": 100, "category": "vegetable"}
{"name": "spinach", "weigth": 50, "category": "vegetable"}
],
}

Answer 1

Using the SQL-Style Operators JOIN and INDEX would be:
{weights: [JOIN(INDEX(.categories[]; .name); .weigths[]; .name; add)]}
{
"weights": [
{
"name": "apple",
"weigth": 200,
"category": "fruit"
},
{
"name": "tomato",
"weigth": 100,
"category": "vegetable"
}
]
}
Demo
For the updated structure, the INDEX of .categories needs to be tweaked (e.g. employing to_entries) in order to provide the .key as .category for each array item:
{weights: [JOIN(
# INDEX(.categories[]; .name); # This has been replaced with:
INDEX(.categories | to_entries[]; .value[]) | .[] |= {category: .key};
.weigths[]; .name; add)]}
{
"weights": [
{
"name": "apple",
"weigth": 200,
"category": "fruit"
},
{
"name": "orange",
"weigth": 300,
"category": "fruit"
},
{
"name": "tomato",
"weigth": 100,
"category": "vegetable"
},
{
"name": "spinach",
"weigth": 50,
"category": "vegetable"
}
]
}
Demo

Answer 2

You may be interested in checking out ~Q (pronounced "unquery"), a query language that I very recently released, that just like jq can run as a command-line tool.
https://github.com/xcite-db/Unquery
In the first case, the query would be:
{
"#var cat:categories[]": {
"$(name)":"category"
},
"#return:weigths[]": [{
"{}:" : ".",
"category":"$var(cat).$(name)"
}]
}
And in the updated example, the query is:
{
"#var cat: categories:{}:[]": {
"$(.)": "$key"
},
"#return:weigths[]": [{
"{}:": ".",
"category":"$var(cat).$(name)"
}]
}
In either case, the trick is to start by generating a map from fruits to categories, and then use this map to add the category to each entry. So the difference is only in the first part (generating the map).
A variable in ~Q can contain any JSON value/document. The code:
"#var cat: categories:{}:[]": {
"$(.)": "$key"
},
Converts categories in-memory into:
{
"apple": "fruit",
"orange": "fruit",
"spinach": "vegetable",
"tomato": "vegetable"
}
And after that, you can use it to retrieve the category with $var(cat).$(name). If you have a different structure for categories, all you need to do is to change that part of the code to generate the mapping from the categories structure.