Extending or Modifying Blacklight Search Behavior

Solr parameters used by for a given request can come from several different places.

Solr request handler: solrconfig.xml in your solr config
Application logic: logic in the BL rails app itself

Solr request handler

The Solr Request Handlers may be configured in the solrconfig.xml and are baked into the request handler itself. Depending on how you have blacklight configured, your app may be using the same Solr request handler for all searches, or may be using different request handlers for different "search fields".

The request handler is often set up with default parameters:

  <requestHandler name="standard" class="solr.SearchHandler" >
     <lst name="defaults">
       <str name="echoParams">explicit</str>
       <str name="rows">10</str> 
       <str name="fl">*</str>   
       <str name="facet">true</str>
       <str name="facet.mincount">1</str>
       <str name="facet.limit">30</str> 
       <str name="facet.field">access_facet</str>
       <str name="facet.field">author_person_facet</str>
       <str name="facet.field">author_other_facet</str>
       <str name="facet.field">building_facet</str>
       <str name="facet.field">callnum_1_facet</str>
       <str name="facet.field">era_facet</str>
       <str name="facet.field">format</str>
       <str name="facet.field">geographic_facet</str>
       <str name="facet.field">language</str>
       <str name="facet.field">pub_date_group_facet</str>
       <str name="facet.field">topic_facet</str>
     </lst>
  </requestHandler>

Configuration

The default application logic (explained below) looks in configuration for things like the name of a the solr request handler to use, and default request parameters to send on every solr search request (or with every request from a certain blacklight search type/field). An example getting started configuration is generally installed into your app when you install Blacklight, at config/initializers/blacklight_config.rb. Or see the current example in blacklight source: [https://github.com/projectblacklight/blacklight/blob/master/config/initializers/blacklight_config.rb]

Nothing special about the filename 'blacklight_config.rb', anything in a Rails app at config/initializers/*.rb gets run on application startup, what matters it the code.

Application logic

The logic Blacklight uses to determine how to map user-supplied parameters into Solr request parameters for a given application request is in the #solr_search_params method in the SolrHelper module. Note that [[CatalogController|https://github.com/projectblacklight/blacklight/blob/master/app/controllers/catalog_controller.rb]] extends [[SolrHelper|https://github.com/projectblacklight/blacklight/blob/master/lib/blacklight/solr_helper.rb]], so the SolrHelper methods become available in the CatalogController (and other controllers, if they extend SolrHelper too).

Behind the scenes, #solr_search_params uses the class_inheritable_accessor method solr_search_params_logic. solr_search_params_logic is essentially a class variable that is mixed into the CatalogController and provides an ordered list of methods to call that may inspect the supplied user parameters and add, remove, or modify the Solr request parameters that will be sent to an RSolr object, which in turn will convert the hash into query parameters for a Solr request. One confusing thing is that RSolr and RSolr-ext provide their own mappings from certain custom terms to Solr-recognized request parameters. For instance, a :per_page key in that hash will get mapped to the Solr &rows parameter -- but a :rows key will also. We Blacklight developers have found that using these special RSolr "aliases" leads to confusion, as well as confusing bugs (if both :per_page and :rows are set in the hash, what happens can be hard to predict). So we try to avoid using the special RSolr aliases, and just use ordinary Solr parameters in our hashes. But you may encounter older code that uses the RSolr aliases.

There can be similar confusing behavior or bugs if one piece of code adds a key to a hash using a :symbol, but another piece of code looks for and/or adds that same key to hash using "string" instead. RSolr accepts either one, but if both are present RSolr behavior can be unexpected. And even before it gets to RSolr, you may have code that thinks it replaced a key but did not becuase it was using the wrong form. Blacklight developers have agreed to try and always use :symbol keys in hashes meant for Solr parameters, to try and avoid these problems. Longer term, we could probably make some code changes to make this kind of error less likely.

The default #solr_search_params_logic is meant to handle very common cases and patterns based on simple configuration options, usually set in the config/blacklight_config.rb file, and the user-supplied URL parameters.

[[Blacklight.config[:default_solr_params]|https://github.com/projectblacklight/blacklight/blob/master/config/initializers/blacklight_config.rb#L44]]
- Default params sent to solr with every search, including the default :qt param, which determines which Solr request handler will be targetted. Some people like to use solrconfig.xml request handler defaults exclusively, and include only a :qt here; others find it more convenient to specify some defaults at the application level here.
[[Blacklight.config[:search_fields]|https://github.com/projectblacklight/blacklight/blob/master/config/initializers/blacklight_config.rb#L194]]
- An array of hashes, each of which defines a 'search field' which will be presented in a select menu in the BL user interface search box. These 'search fields' don't neccesarily correspond 1-to-1 with Solr fields. What they instead correspond to is Solr parameter over-rides that will be used for this BL UI search field. Those over-rides are present here.
- You could simply chose a different :qt Solr request handler for each search field, which has it's own default parameters in the sorlconfig.xml. This is what we started out doing with Solr, but found it led to much duplication of information in solrconfig.xml.
- You can continue using the same Solr request handler, but simply specify different parameters which will be included in the http query string sent to Solr here, with the :solr_parameters key. This works fine, but some people don't like how it makes your Solr requests much longer/more complex in the Solr logs; and/or they prefer to control this Solr side instead of Application side.
- For the best of both worlds, although it's a bit confusing at first, you can use the :solr_local_parameters key to have parameters supplied to Solr using the Solr LocalParams syntax, which means you can use "parameter dereferencing" with dollar-sign-prefixed references to variables defined in solrconfig.xml request handler. This is what the current example BL setup does.

So the default implementation of #solr_search_params takes these configured parameters, combines them with certain query parameters from the current users HTTP request to the BL app, and prepares a Hash of parameters that will be sent to solr. For common use patterns, this is all you need.

But sometimes you want to add some custom logic to #solr_search_params to come up with the solr params Hash in different ways. Typically to support a new UI feature of some kind, either in your local app or in a Blacklight add-on plugin you are developing.

Extending Blacklight::SolrHelper#solr_search_params

The approach described here was introduced in Blacklight 2.9 (unreleased). For earlier releases of Blacklight, please see the documentation for Blacklight 2.8 or earlier.

To add new search behaviors (e.g. authorization controls, pre-parsing query strings, etc), the easiest route is to add additional steps to the #solr_search_params_logic, either at the beginning of the list (to set default parameters) or at the end of the list (to force particular parameters). You can add custom steps within the blacklight_config.rb (or in many other places) by adding a Symbol with the name of the method you wish to use, e.g.:

CatalogController.solr_search_params_logic << :show_only_public_records

Because #solr_search_params_logic is just an ordinary array, you may perform any normal array operation (e.g. push/pop/delete/insert) to customize the parameter generation to meet your needs.

#solr_search_params_logic steps take two inputs, the hash of solr_parameters and the hash of user_parameters (often provided by the URL parameters), and modifies the solr_parameters directly, as needed.

./app/controllers/catalog_controller.rb

require_dependency 'vendor/plugins/blacklight/app/controllers/catalog_controller.rb'
class CatalogController < ApplicationController 
  include MyApplication::SolrHelper::Authz
end

./lib/my_application/solr_helper/authz.rb

module MyApplication::SolrHelper::Authz
  def show_only_public_records solr_parameters, user_parameters
    solr_parameters[:fq] ||= []
    solr_parameters[:fq] << 'public_b:1'
  end
end

Common Pitfalls

When extending the Blacklight CatalogController (or other core Blacklight classes), as described in the above example, it's important to pull in the existing code using 'require_dependency' rather than simply 'require' which will only allow for adding new methods; attempts to override existing methods will fail.