Is it possible to create subcategories within subcategories? For example, can I have a grandparent category, parent subcategory, and child subcategory? If not, is there any alternative to creating a separate instance for each “grandparent”?
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No, there are only two levels of categories in Discourse.
We’ll combine the 2 levels of categories with tags. It’s not just about taxonomy, it’s also about moderation rights.
Will 3 levels of categorization be possible in the future? My site is about 4 sports, then within each sport, I want a category called “Gear”, and then within “Gear” have the different brands.
I could also do a top level category called Gear, and then have the Gear Companies underneath.
Or just have the Gear companies for each sport listed directly under the sport.
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No plans for more than 2 levels of categories. As @jeans noted, adding tags into the mix makes more sense at that point. So use the official tag support built into Discourse.
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@codinghorror What would be the technical implications of allowing deeper nesting of categories? I don’t have a specific requirement in mind, just curious. I can see UI concerns, but the data model looks like it could handle chains of parents.
Here a typical case where we need to configure Discourse with 3 levels,
Local Groups - Open Knowledge Forums
the local-groups are in fact the “level zero”, in some cases we also change language (!), as at local-groups/okbr. The main topics are at this level-2, it is ok, but…
The level-3 is important to isolate very distinct topics and/or communities:
-
to isolate official topics. Example: today we use the “official” tag as here, but we need to isolate as
local-groups/okbr/oficial, -
to isolate projects, because they have independent communities and very specific topics. Example: this project was losted in an external category, openspending/gastos-abertos, we need an intersection between
openspendingandlocal-groups/okbrand we understand that the second is the priority, so need categorylocal-groups/okbr/prj-gastos-abertosthat is a typical “local group project” category.
This is interesting that have you decided that any existing field of knowledge may require to be split by exactly one level of categories but never by level two or higher. While I don’t know your exact reasoning but for me it’s very hard to believe that this can be the truth.
Two levels, not three. And we’re not the only people to think so.
We were very proud of our user interface and the fact that we had a way to browse 16,000 (!!) pages of documentation on a CD-ROM. But browsing the hierarchy felt a little complicated to us. So we asked Tufte to come in and have a look, and were hoping perhaps for a pat on the head or some free advice.
He played with our AnswerBook for about 90 seconds, turned around, and pronounced his review:
“Dr Spock’s Baby Care is a best-selling owner’s manual for the most complicated ‘product’ imaginable – and it only has two levels of headings. You people have 8 levels of hierarchy and I haven’t even stopped counting yet. No wonder you think it’s complicated.”
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How can you actually tag a category?
Hello , thank you for the great discourse tool and community. I would like to be informed if there is the possibility to create sub sub categories dynamically? How can this be done with tags?
Try this
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You can create subcategories now, just make the parent category the category that you want to have the subcategory under
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It is possible to change the level of subcategories now through a hidden site setting:
cd /var/discourse/
./launcher enter app
rails c
SiteSetting.max_category_nesting = 3
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He’s referring to sub sub categories ie a 3rd level, not a 2nd level (sub categories).
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Right. And that hidden setting makes it possible to have Sub-Sub-categories (three levels, hence the 3 in the setting). It’d been around for a year, I think, but it didn’t get much fanfare when it was added (it just gave been in the release notes, but maybe didn’t get a heading). I guess some enterprise customer must have wanted the feature, but they still think it’s a bad idea. 
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I have never tried creating a sub sub category, is it not possible through the UI?
It it is possible once the feature is enabled. You set the parent to the sub-category instead of the parent category to create a sub-sub.
This what the create a category screen looks like when making a sub-sub:
I don’t remember who asked for it originally, but we asked to have it enabled a little over a year ago on the NaNoWriMo forums in hopes it would would eventually help solve a much larger and somewhat unique issue we have.
It solved our test-case issue but sadly nothing short of a miracle would ever fix the other issue. 
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