I wrote this plugin way back in BL 2.x days, but I think many don’t know about it, and I don’t think anyone but me is using it, so I thought I’d take the opportunity having updated it, to advertise it.
blacklight_cql gives your BL app the ability to take CQL queries as input. CQL is a query language for writing boolean expressions (http://www.loc.gov/standards/sru/cql/); I don’t personally consider it suitable for end-users to enter manually, and don’t expose it that way in my BL app.
But I do it use it as an API for other internal software to make complex boolean queries against my BL app; like “format = ‘Journal’ AND (ISSN = X OR ISSN =Y OR ISBN = Z)” Paired with the BL Atom response, it’s a pretty powerful query API against a BL app.
Both direct Solr fields, and search_fields you’ve configured in Blacklight are available in CQL; they can even be mixed and matched in a single query.
The blacklight_cql plug-in also provides an SRU/ZeeRex EXPLAIN handler, for a machine-readable description of what search fields are supported via CQL. Here’s “EXPLAIN” on my server: https://catalyst.library.jhu.edu/catalog/explain
The plug-in does NOT provide a full SRU/SRW implementation — but as it does provide some of the hardest parts of an SRW implementation, it would probably not be too hard to write a bit more glue code to get a full implementation. I considered doing that to make my BL app a target of various federated search products that speak SRW, but never wound up having a business case for it here. (Also, it may or may not actually work out, as SRW tends to vary enough that even if it’s a legal-to-spec SRW implementation, that’s no guarantee it will work with a given client).
Even though the blacklight_cql plugin has been around for a while, it’s perhaps still somewhat immature software (or maybe it’s that it’s “legacy” software now?). It’s worked out quite well for me, but I’m not sure anyone else has used it, so it may have edge case bugs I’m not running into, or bugs that are triggered by use cases other than mine. It’s also, I’m afraid, not very well covered by automated tests. But I think what it does is pretty cool, and if you have a use for what it does, starting with blacklight_cql should be a lot easier than starting from scratch.
Feel free to let me know if you have questions or run into problems.