metabase.models.json-migration

Set the updated version if the column-value has data. Doesn't do anything if it's empty since empty values are assumed to result in version-appropriate default behavior and don't need an explicit version key.

Create a multi-method with the given name that will perform JSON migrations. Individual cases (with appropriate logic!) must be defined by the user. The resulting multi-method accepts two arguments: the value of the column and the desired version. Versioning is assumed to start at 1 and be stored in the JSON blob under the :version key (and no version at all is assumed to be 1 as well). Updating the version is not handled here; in practice you should probably chain the migration method together with update-version (defined above). Non-upgrades (e.g., upgrading a value from version 2 to version 2) are handled and treated as a no-op.

For example, imagine a User model with a JSON column called login_settings. This originally contained a boolean key remember_me that persisted a session for 30 days, but the number of days is now configurable per user. The migration code would look like this:

(def login-settings-version 2)

(def-json-migration migrate-login-settings*)

(defmethod migrate-login-settings* [1 2] [login-settings _version]
  (assoc login-settings :remember_me_days (if (:remember_me login-settings) 30 0)))

(defn migrate-login-settings
  [login-settings] ;; note that this only takes the one argument, not two
  (-> login-settings
      (migrate-login-settings* login-settings-version)
      (update-version login-settings-version)))

(migrate-login-settings {:remember_me true})                 ;; => {:remember_me_days 30, :version 2, :remember_me true}
(migrate-login-settings {:remember_me false})                ;; => {:remember_me_days 0,  :version 2, :remember_me false}
(migrate-login-settings {:remember_me true, :version 1})     ;; => {:remember_me_days 30, :version 2, :remember_me true}
(migrate-login-settings {:remember_me_days 15, :version 2})  ;; => {:remember_me_days 15, :version 2}