BUG: Using post_tag in query is setting the slug param in a wrong format

[pabamato]

Describe the bug

From the WP_Query docs, if you use slugs as the matching param, you can pass params in two formats:

• Array of slugs
• String of comma separated slugs

On https://github.com/10up/ElasticPress/blob/trunk/includes/classes/Indexable/Post/Post.php#L844,
The params gets converted to an array, so looks like you can not query for multiple values.

Same wrong format on https://github.com/10up/ElasticPress/blob/trunk/includes/classes/Indexable/Post/Post.php#L852
Also, looks like this is being added with the correct format later on https://github.com/10up/ElasticPress/blob/trunk/includes/classes/Indexable/Post/Post.php#L906
'post_tag' is the query_var for that particular taxonomy

Steps to Reproduce

Query posts passing multiple post_tags:
Having a site with 2 posts(post1, post2) and 2 terms in post_tags(slug1,slug2):
post1 ->slug1
post2 ->slug2

$args_for_multiple_slugs_array_format = [
	'post_tag' => [
		'slug1',
		'slug2',
	],
];
$args_for_multiple_slugs_string_formnat = [
	'post_tag' => 'slug1,slug2',
];
$posts_query = new \WP_Query( $args_for_multiple_slugs_array_format );
$posts_query = new \WP_Query( $args_for_multiple_slugs_string_formnat );

Expected behavior
Both queries should return 2 posts since it should match posts having either slug1 or slug2.

Screenshots

Environment information

• Device: MacBook
• OS: MacOS 11.14.0
• Browser and version: any
• WordPress version: 5.8

• ElasticPress version: 3.6.1
• Elasticsearch version: 5.6.16
• Where do you host your Elasticsearch server? 10updocker
• What ElasticPress features do you have currently active? -
• Plugins and version:N/A
• Theme and version: N/A

• Site Health Info:

Additional context

[pabamato]

Adding additional details for replicating the issue:

• Default WordPress install with twenty-twentyone theme.
• Filtering the main query and passing post_tag is causing an error in the ElasticPress endpoint
  If I pass post_tag within 'tax_query', is working as expected

[pabamato]

If that helps to speed-up the debug process, I created a gist having a site export xml file and part of the code I'm using:
https://gist.github.com/pabamato/9825b1c1494eb10f44c4619529196e1b

[oscarssanchez]

Hey @pabamato did

$args_for_multiple_slugs_array_format = [
	'ep_integrate' => true,
	'post_tag' => [
		'slug1',
		'slug2',
	],
];

$posts_query = new \WP_Query( $args_for_multiple_slugs_array_format );

work for you?

When I tested this I got a 400 with this error:

{
    "error": {
        "root_cause": [
            {
                "type": "parsing_exception",
                "reason": "[terms] unknown token [END_ARRAY] after [terms.post_tag.slug]",
                "line": 1,
                "col": 312
            }
        ],
        "type": "x_content_parse_exception",
        "reason": "[1:312] [bool] failed to parse field [must]",
        "caused_by": {
            "type": "x_content_parse_exception",
            "reason": "[1:312] [bool] failed to parse field [must]",
            "caused_by": {
                "type": "parsing_exception",
                "reason": "[terms] unknown token [END_ARRAY] after [terms.post_tag.slug]",
                "line": 1,
                "col": 312
            }
        }
    },
    "status": 400
}

This seems to be because of bad formatting from the plugin. I just want to check if it is something that recently broke. From the WP_Query docs you linked https://developer.wordpress.org/reference/classes/wp_query/#tag-parameters it seems that the correct param is "tag" right? so something like this:

$args_for_multiple_slugs_array_format = [
	'ep_integrate' => false,
	'tag' => [
		'slug1',
		'slug2',
	]
];

Using post_tag as a query paremeter did not work for me, in this case not using elasticpress, but tag did. I could confirm that a) comma separated string doesn't work, but also that b) an array with more than one element also does not bring the results WP_Query brings

[oscarssanchez]

Once merged #2341 should address the identified problems.