Indexing Hierarchical Taxonomies

When you display a hierarchical facet in Appkit, Appkit initially only shows the top-level nodes in the taxonomy and asynchronously fetch nodes further down the tree by using Solr facet prefix queries as each node gets expanded. This greatly reduces the size of the taxonomy that would have to be fetched initially, if you had to request the whole tree and render it on the page all at once. This means that, irrespective of how deep and wide your taxonomy is, it can be represented in the user interface in a performant manner.

However, when using only one categorisation field, Appkit does not know whether to show a link to expand each node unless it looks ahead and does another Solr query to check whether a given node has any children. To address this, you can further augment the information Appkit indexes for each document that is tagged with hierarchical categories, so that Appkit can quickly look up both a single tree level (for example, facet.prefix = 0/) and also determine which of the nodes at a given level have children (using a single facet query).

More specifically, for each hierarchical facet my_facet, you create an additional meta-facet named my_facet_parents that contains information about the taxonomy categories that have children (that is, those nodes that are not leaf nodes in the hierarchy). Using the example from above, you would index location in exactly the same way as before. In addition, you would index these terms for the new location_parents meta-facet:

With this scheme in place, Appkit will automatically generate and run a single facet query to retrieve all top-level nodes and identify those nodes that have children.