datafy, nav, and the :schema option

Clojure 1.10 introduced a new namespace, clojure.datafy, and two new protocols (Datafiable and Navigable) that allow for generalized, lazy navigation around data structures. Cognitect also released REBL -- a graphical, interactive tool for browsing Clojure data structures, based on the new datafy and nav functions.

Shortly after REBL's release, I added experimental support to clojure.java.jdbc for datafy and nav that supported lazy navigation through result sets into foreign key relationships and connected rows and tables. next.jdbc bakes that support into result sets produced by execute! and execute-one!.

The datafy/nav Lifecycle

Here's how the process works:

• execute! and execute-one! produce result sets containing rows that are Datafiable,
• Tools like REBL can call datafy on result sets to render them as "pure data" (which they already are, but this makes them also Navigable),
• Tools like REBL allow users to "drill down" into elements of rows in the "pure data" result set, using nav,
• If a column in a row represents a foreign key into another table, calling nav will fetch the related row(s),
• Those can in turn be datafy'd and nav'd to continue drilling down through connected data in the database.

Identifying Foreign Keys

By default, next.jdbc assumes that a column named <something>id or <something>_id is a foreign key into a table called <something> with a primary key called id. As an example, if you have a table address which has columns id (the primary key), name, email, etc, and a table contact which has various columns including addressid, then if you retrieve a result set based on contact, call datafy on it and then "drill down" into the columns, when (nav row :contact/addressid v) is called (where v is the value of that column in that row) next.jdbc's implementation of nav will fetch a single row from the address table, identified by id matching v.

You can override this default behavior for any column in any table by providing a :schema option that is a hash map whose keys are column names (usually the table-qualified keywords that next.jdbc produces by default) and whose values are tuples containing the name of the table to which that column is a foreign key and the name of the key column within that table. These tuples can optionally include a third value which indicates the cardinality of the foreign key relationship: :one or :many. The default is :one and indicates a one-to-one or many-to-one relationship -- navigation will produce a single row. :many indicates a one-to-many or many-to-many relationship -- navigation will produce a result set.

The default behavior in the example above is equivalent to this :schema value:

{:contact/addressid [:address :id :one]} ; :one is the default and could be omitted

If you had a table to track the current valid/bouncing status of email addresses, where email is the primary key, you could provide automatic navigation into that using:

{:contact/addressid [:address :id :one]
 :address/email [:deliverability :email]}

If you use foreign key constraints in your database, you could probably generate this :schema data structure automatically from the metadata in your database.

Behind The Scenes

Making rows datafiable is implemented by adding metadata to each row with a key of clojure.core.protocols/datafy and a function as the value. That function closes over the connectable and options passed in to the execute! or execute-one! call that produced the result set containing those rows.

When called (datafy on a row), it adds metadata to the row with a key of clojure.core.protocols/nav and another function as the value. That function also closes over the connectable and options passed in.

When that is called (nav on a row, column name, and column value), if a :schema entry exists for that column or it matches the default convention described above, then it will fetch row(s) using next.jdbc's Executable functions -execute-one or -execute-all, passing in the connectable and options closed over.

The protocol next.jdbc.result-set/DatafiableRow has a default implementation of datafiable-row for clojure.lang.IObj that just adds the metadata to support datafy. There is also an implementation baked into the result set handling behind plan so that you can call datafiable-row directly during reduction and get a fully-realized row that can be datafy'd (and then navigated).

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