How do I make a Hasura data API query to fetch rows based on the length of the their array relationship's value? - hasura

Question

Referring to the default sample schema mentioned in https://hasura.io/hub/project/hasura/hello-world/data-apis i.e. to the following two tables:
1) author: id,name
2) article: id, title, content, rating, author_id
where article:author_id has an array relationship to author:id.
How do I make a query to select authors who have written at least one article? Basically, something like select author where len(author.articles) > 0

Answer 1

TL;DR:
There's no length function that you can use in the Hasura data API syntax right now. Workaround 1) filter on a property that is guaranteed to be true for every row. Like id > 0. 2) Build a view and expose APIs on your view.
Option 1:
Use an 'always true' attribute as a filter.
{
"type": "select",
"args": {
"table": "author",
"columns": [
"*"
],
"where": {
"articles": {
"id": {
"$gt": "0"
}
}
}
}
}
This reads as: select all authors where ANY article has id > 0
This works because id is an auto-incrementing int.
Option 2:
Create a view and then expose data APIs on them.
Head to the Run SQL window in the API console and run a migration:
CREATE VIEW author_article_count as (
SELECT au.*, ar.no_articles
FROM
author au,
(SELECT author_id, COUNT(*) no_articles FROM article GROUP BY author_id) ar
WHERE
au.id = ar.author_id)
Make sure you mark this as a migration (a checkbox below the RunSQL window) so that this gets added to your migrations folder.
Now add data APIs to the view, by hitting "Track table" on the API console's schema page.
Now you can make select queries using no_articles as the length attribute:
{
"type": "select",
"args": {
"table": "author_article_count",
"columns": [
"*"
],
"where": {
"no_articles": {
"$gt": "0"
}
}
}
}