Where clause for JSON field on array of objects - mysql

Question

Working with MySQL 5.7.19, I swear an hour ago this worked, but now im getting nothing returned from my query
CREATE TABLE FlattenedData.blog_posts
(
post_id CHAR(13) NOT NULL PRIMARY KEY UNIQUE,
post_data JSON,
date_published DATETIME NOT NULL, # for primary indexing
date_added DATETIME DEFAULT CURRENT_TIMESTAMP,
date_updated DATETIME DEFAULT CURRENT_TIMESTAMP on UPDATE CURRENT_TIMESTAMP,
post_categories VARCHAR(255) GENERATED ALWAYS AS (post_data->>"$.categories[*].slug"),
post_tags VARCHAR(512) GENERATED ALWAYS AS (post_data->>"$.tags[*].slug"),
KEY idx_date_published (date_published),
INDEX idx_categories (post_categories),
INDEX idx_tags (post_tags),
INDEX idx_categories_tags (post_categories, post_tags)
);
as a note: a post can have multiple categories, just like tags
Here's my query
select
*
from blog_posts
WHERE
post_data->>"$.categories[*].slug" = "site-news"
Like I said, I swear this was working earlier, but now I get nothing back.
Here's the explain:
Even if I delete the indexes and generated columns and just use plain json field, I still suddenly get nothing. The only thing I can use that gets results, is JSON_SEARCH, but there's thousands of records and these are rather large json blobs, and it's possible the searched text can show up in the body
btw, a category field looks like this
[{slug: "site-news", title: "Site News"}, {slug: "personal", title: "Personal"}]
tags follow exactly the same structure
EDIT
I just tried with post_data->>"$.categories[0].slug" = "site-news" and that brought in records. But I need the where clause to take into consideration all elements of the array as I cannot guarantee the array element slot this category is going to be in.
As MySQL's docs state: [*] represents the values of all cells in the array. https://dev.mysql.com/doc/refman/5.7/en/json-path-syntax.html
A core problem of using
select
*
from blog_posts
WHERE
JSON_CONTAINS(post_data->"$.categories[*].slug", json_quote("site-news"))
is that it completely avoids using my indexes, which is going to be key here

Answer 1

Your path expression with wildcard returns an array of values, as an array in JSON notation.
SELECT post_data->>'$[*].slug' FROM blog_posts;
+---------------------------+
| post_data->>'$[*].slug' |
+---------------------------+
| ["site-news", "personal"] |
+---------------------------+
That's clearly not equal to the scalar string 'site-news'.
So you an use JSON_SEARCH() on the JSON array to find a specific string:
SELECT * FROM blog_posts
WHERE JSON_SEARCH(post_data->>'$[*].slug', 'one', 'site-news') IS NOT NULL;
I tested that with MySQL 8.0.3-rc. I loaded this data:
INSERT INTO blog_posts (post_id, date_published, post_data)
VALUES('blah blah', now(), '[{"slug": "site-news", "title": "Site News"}, {"slug": "personal", "title": "Personal"}]');
I know this isn't the format of your post_data, but it still demonstrates that using a wildcard path on JSON returns an array.