Appending Geojson with json field using JQ - json

Question

I have a project I'm working on that creates a choropleth map with all US county borders loaded from file1.json and filled with a color gradient based on values in file2.json. In previous iterations, I just enter values manually into file1.json, but now I want to expand my map and make it more user-friendly.
file1.json is structured like this:
{
"type": "FeatureCollection",
"features": [
{
"type": "Feature",
"properties": {
"GEO_ID": "0500000US06001",
"STATE": "06",
"COUNTY": "001",
"NAME": "Alameda",
"LSAD": "County",
"CENSUSAREA": 739.017
},
"geometry": {
"type": "Polygon",
"coordinates": [
[
[
-122.30936,
37.77615
],
[
-122.317215,
37.778527
]
]
]
}
},
...
]
}
file2.json is structued like this:
[
{
"County": "Alameda",
"Count": 25
},
{
"County": "Amador",
"Count": 1
},
{
"County": "Butte",
"Count": 2
},
...
]
I want to create a new file that includes everything from file1.json, but append it to include the relevent Count field based on the County field.
The result would look like this:
[
{
"type": "Feature",
"properties": {
"GEO_ID": "0500000US06001",
"STATE": "06",
"COUNTY": "001",
"NAME": "Alameda",
"Count": "25",
"LSAD": "County",
"CENSUSAREA": 739.017
},
"geometry": {
"type": "Polygon",
"coordinates": [
[
[
-122.30936,
37.77615
],
[
-122.317215,
37.778527
]
]
]
}
},
...
]
I'm new to using jq, but I've played around with it enough to get it running in PowerShell.

Answer 1

Here is a test.jq file which may help
# utility to create lookup table from array of objects
# k is the name to use as the key
# f is a function to compute the value
#
def obj(k;f): reduce .[] as $o ({}; .[$o[k]] = ($o | f));
# create map from county to count
( $file2 | obj("County";.Count) ) as $count
# add .properties.Count to each feature
| .features |= map( .properties.Count = $count[.properties.NAME] )
Example use assuming suitable file1.json and file2.json:
$ jq -M --argfile file2 file2.json -f test.jq file1.json
{
"type": "FeatureCollection",
"features": [
{
"type": "Feature",
"properties": {
"GEO_ID": "0500000US06001",
"STATE": "06",
"COUNTY": "001",
"NAME": "Alameda",
"LSAD": "County",
"CENSUSAREA": 739.017,
"Count": 25
},
"geometry": {
"type": "Polygon",
"coordinates": [
[
[
-122.30936,
37.77615
],
[
-122.317215,
37.778527
]
]
]
}
}
]
}
Try it online!
I notice that "Count" is a string in your example output but it's a number in the sample file2. If you need to convert that to a string you can include a call to tostring. e.g.
.features |= map( .properties.Count = ( $count[.properties.NAME] | tostring ) )
or you could perform the conversion when the lookup table is created, e.g.
( $file2 | obj("County"; .Count | tostring ) ) as $count