Allow users to tag documents

[lukavdplas]

Allow users to assign tags to documents.

The document view should include a component to edit the document's tags. Also, there should be some kind of interface where users can edit their own tags and see their tagged documents.

Since tags are personal, it probably makes the most sense to store them in the SQL database with the document ID and corpus name. However, this makes it difficult to use elasticsearch functionality with tags, such as implementing tags as a filter.

• Models for tagging #1147
• Backend support for searching with tags #1148
• Views for user tag management #1149
• Tag overview in frontend #1151
• Include tags in search history #1152
• User Tag limits #1211
• Add a default tag #1216
• Tag documents in interface #1150
• visualisations / downloads with tags #1234
• filter widget for tags #1235

[lukavdplas]

We've been discussing potential uses for tags. Perhaps a few common uses would be the following:

• Bookmark a small number of relevant documents for close reading or downloading.
• Tag everything matching a query as, for instance, 'mentions democracy'. Then, when you browse other queries in the future, you can immediately see this tag on some documents, giving you a quick indication of relevant properties or topics.
• Tag a complex query, then use the tag as a filter in future queries. For instance, filter all social-democratic parties in the Dutch parliamentary debates and tag them 'social democrat'. Now future searches for social democrats are a lot quicker.

The latter two are a bit tricky if we implement tags by storing document IDs in the SQL database. Tagging a few queries can result in millions of (user, document, tag) datarows. Using a tag as a filter is also tricky since elasticsearch doesn't have access to the tags.

It may be better to store tags not as document IDs, but as a queries. If the user tags a single document, this is just a query for that document ID. To get all documents with a tag, you just have to get the union of all the queries that are assigned to it.