Listing associated IDs during a Controller query in CakePHP 3

Yesterday I looked into adding a key-value pair such as  "tag_ids": [1, 2, 3] to a JSON object returned by a serialized CakePHP view, for Bookmarks belonging to many Tags. My solution produces the desired result, but involves the execution of a new database query for every bookmark, on top of the one that retrieved the record in the first place. This is not exactly desirable behaviour, so today I looked into other options. I still don’t have what I consider to be an optimal solution, but did come up with an alternative that manages to pull all the associated Tag ids in the same query that retrieves the current batch of Bookmarks.

There is a new disadvantage introduced by this solution, because the resulting KVP contains a string instead of an array, looking like  "tag_ids": "[1, 2, 3]" . Additionally, the code is nowhere near ready to be generalized for easy addition to multiple controllers, nor even particularly elegant in terms of making use of Cake’s routines for inflection etc. Nevertheless, I want to record it while it’s still fresh in my mind.

The idea is to use the CakePHP query builder to LEFT JOIN the Bookmarks model’s table to the join table that contains its many-to-many relationships with the Tags model. The query is then grouped by all of the Bookmarks fields, and the  tag_id field from the join table is aggregated into a single comma-delimited string. I’m using PostgreSQL, so accomplish this with the string_agg command. Here’s a working example:

I found that I had to set autofields(false)  to avoid having CakePHP automatically include every field in the join table, and therefore needing to add them to the group-by clause or aggregating them in some fashion.

There are a few improvements that can be made right away. Additional joins can be added, but will introduce multiples in the concatenated string unless the string aggregation is made distinct,  string_agg(distinct Book...). Bookmarks with no tags yield   "tag_ids": null, but can be made to give "tag_ids": "[]"  by wrapping the aggregation in coalesce(..., '').

Larger-scale improvements could involve outputting an array instead of a string in the JSON, perhaps by returning a postgres array instead of a CSV string, and teaching Cake how to deal with that properly. Beyond that, not hard-coding the table names, generating the list of joins automatically, and generally wrapping this all up into a behaviour or trait would be nice steps to take.

Array of associated IDs in CakePHP 3.0 “belongsToMany” relationship

Today I was struggling with how to get CakePHP to return a JSON representation of a model, including a simple array of the foreign-key ids from the join table that specifies a mutual belongsToMany relationship (formerly, hasAndBelongsToMany or HABTM). For a concrete example, I wanted to build on the Bookmarker tutorial by creating an API endpoint to retrieve bookmarks, each containing an array of its tag ids. Something like this:

Using Cake’s data views via the RequestHandler and _serialize elements made serving the JSON straightforward enough for the Bookmark model without the tags. Adding the tags to the output was easy enough using  contain() to retrieve associated data. This lead to having the entire tag included in the result though, not the compact “tag_ids” array I had in mind. Even selecting only the id field and setting autofields(false) left an array of objects, including extraneous join information. Instead of containing integers, the tags array of each bookmark contained objects that looked like this,

where a simple  1 was all I wanted.

To solve this problem, I ended up using a virtual field on the Bookmark model that creates the desired array of ids, and which can be easily serialized to JSON.

First, as with other approaches to the data view, the RequestHandler had to be added to either the Bookmarks controller or the App controller.

Next add the virtual tag_ids field through the magic method _getTagIds(), which queries the join table Bookmarks_Tags to select the tag_id for every tag associated with the current bookmark_id. This list is then used to populate a standard PHP array of the integer ids, which becomes the value of the virtual field.

Then all it took in the Bookmarks controller was to query for the additional non-virtual fields to be included, and store the results in a serialized variable:


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