Following is a tutorial on how to use the Mito ORM.

As usual, this is best read on the Common Lisp Cookbook. It will be updated there.

The Database section on the Awesome-cl list is a resource listing popular libraries to work with different kind of databases. We can group them roughly in four categories:

  • wrappers to one database engine (cl-sqlite, postmodern, cl-redis,…),
  • interfaces to several DB engines (clsql, sxql,…),
  • persistent object databases (bknr.datastore (see chap. 21 of “Common Lisp Recipes”), ubiquitous,…),
  • Object Relational Mappers (Mito),

and other DB-related tools (pgloader).

We’ll begin with an overview of Mito. If you must work with an existing DB, you might want to have a look at cl-dbi and clsql. If you don’t need a SQL database and want automatic persistence of Lisp objects, you also have a choice of libraries.

The Mito ORM and SxQL

Mito is in Quicklisp:

(ql:quickload :mito)


Mito is “an ORM for Common Lisp with migrations, relationships and PostgreSQL support”.

  • it supports MySQL, PostgreSQL and SQLite3,
  • when defining a model, it adds an id (serial primary key), created_at and updated_at fields by default like Ruby’s ActiveRecord or Django,
  • handles DB migrations for the supported backends,
  • permits DB schema versioning,
  • is tested under SBCL and CCL.

As an ORM, it allows to write class definitions, to specify relationships, and provides functions to query the database. For custom queries, it relies on SxQL, an SQL generator that provides the same interface for several backends.

Working with Mito generally involves these steps:

  • connecting to the DB
  • writing CLOS classes to define models
  • running migrations to create or alter tables
  • creating objects, saving same in the DB,

and iterating.

Connecting to a DB

Mito provides the function connect-toplevel to establish a connection to RDBMs:

(mito:connect-toplevel :mysql :database-name "myapp" :username "fukamachi" :password "c0mon-1isp")

The driver type can be of :mysql, :sqlite3 and :postgres.

With sqlite you don’t need the username and password:

(connect-toplevel :sqlite3 :database-name "myapp")

As usual, you need to create the MySQL or Postgre database beforehand. Refer to their documentation.

Connecting sets mito:*connection* to the new connection and returns it.

Disconnect with disconnect-toplevel.

=> you might make good use of a wrapper function:

(defun connect ()
  "Connect to the DB."
  (connect-toplevel :sqlite3 :database-name "myapp"))


Defining models

In Mito, you can define a class which corresponds to a database table by specifying (:metaclass mito:dao-table-class):

(defclass user ()
  ((name :col-type (:varchar 64)
         :initarg :name
         :accessor user-name)
   (email :col-type (or (:varchar 128) :null)
          :initarg :email
          :accessor user-email))
  (:metaclass mito:dao-table-class))

Note that the class automatically adds some slots: a primary key named id if there’s no primary keys, created_at and updated_at for recording timestamps. To disable these behaviors, specify :auto-pk nil or :record-timestamps nil to the defclass forms.

You can inspect the new class:

(mito.class:table-column-slots (find-class 'user))

The class inherits mito:dao-class implicitly.

(find-class 'user)

(c2mop:class-direct-superclasses *)

This may be useful when you define methods which can be applied for all table classes.

For more information on using the Common Lisp Object System, see the clos page.

Creating the tables

After defining the models, you must create the tables:

(mito:ensure-table-exists 'user)

So a helper function:

(defun ensure-tables ()
  (mapcar #'mito:ensure-table-exists '(user foo bar)))

See Mito’s documentation for a couple more ways.

When you alter the model you’ll need to run a DB migration, see the next section.


Fields types

Field types are:

(:varchar <integer>) ,

:serial, :bigserial, :integer, :bigint, :unsigned,

:timestamp, :timestamptz,


Optional fields

Use (or <real type> :null):

   (email :col-type (or (:varchar 128) :null)
          :initarg :email
          :accessor user-email))

Field constraints

:unique-keys can be used like so:

(defclass user ()
  ((name :col-type (:varchar 64)
         :initarg :name
         :accessor user-name)
   (email :col-type (:varchar 128)
          :initarg :email
          :accessor user-email))
  (:metaclass mito:dao-table-class)
  (:unique-keys email))

We already saw :primary-key.

You can change the table name with :table-name.


You can define a relationship by specifying a foreign class with :col-type:

(defclass tweet ()
  ((status :col-type :text
           :initarg :status
           :accessor tweet-status)
   ;; This slot refers to USER class
   (user :col-type user
         :initarg :user
         :accessor tweet-user))
  (:metaclass mito:dao-table-class))

(table-definition (find-class 'tweet))
;        status TEXT NOT NULL,
;        user_id BIGINT NOT NULL,
;        created_at TIMESTAMP,
;        updated_at TIMESTAMP
;    )>)

Now you can create or retrieve a TWEET by a USER object, not a USER-ID.

(defvar *user* (mito:create-dao 'user :name "Eitaro Fukamachi"))
(mito:create-dao 'tweet :user *user*)

(mito:find-dao 'tweet :user *user*)

Mito doesn’t add foreign key constraints for refering tables.


A one-to-one relationship is simply represented with a simple foreign key on a slot (as :col-type user in the tweet class). Besides, we can add a unicity constraint, as with (:unique-keys email).

One-to-many, many-to-one

The tweet example above shows a one-to-many relationship between a user and his tweets: a user can write many tweets, and a tweet belongs to only one user.

The relationship is defined with a foreign key on the “many” side linking back to the “one” side. Here the tweet class defines a user foreign key, so a tweet can only have one user. You didn’t need to edit the user class.

A many-to-one relationship is actually the contraty of a one-to-many. You have to put the foreign key on the approriate side.


A many-to-many relationship needs an intermediate table, which will be the “many” side for the two tables it is the intermediary of.

And, thanks to the join table, we can store more information about the relationship.

Let’s define a book class:

(defclass book ()
       :col-type (:varchar 128)
       :initarg :title
       :accessor title)
       :col-type (or (:varchar 128) :null)
       :initarg :ean
       :accessor ean))
    (:metaclass mito:dao-table-class))

A user can have many books, and a book (as the title, not the physical copy) is likely to be in many people’s library. Here’s the intermediate class:

(defclass user-books ()
      :col-type user
      :initarg :user)
      :col-type book
      :initarg :book))
    (:metaclass mito:dao-table-class))

Each time we want to add a book to a user’s collection (say in a add-book function), we create a new user-books object.

But someone may very well own many copies of one book. This is an information we can store in the join table:

(defclass user-books ()
      :col-type user
      :initarg :user)
      :col-type book
      :initarg :book)
    ;; Set the quantity, 1 by default:
      :col-type :integer
      :initarg :quantity
      :initform 1
      :accessor quantity))
    (:metaclass mito:dao-table-class))

Inheritance and mixin

A subclass of DAO-CLASS is allowed to be inherited. This may be useful when you need classes which have similar columns:

(defclass user ()
  ((name :col-type (:varchar 64)
         :initarg :name
         :accessor user-name)
   (email :col-type (:varchar 128)
          :initarg :email
          :accessor user-email))
  (:metaclass mito:dao-table-class)
  (:unique-keys email))

(defclass temporary-user (user)
  ((registered-at :col-type :timestamp
                  :initarg :registered-at
                  :accessor temporary-user-registered-at))
  (:metaclass mito:dao-table-class))

(mito:table-definition 'temporary-user)
;=> (#<SXQL-STATEMENT: CREATE TABLE temporary_user (
;        name VARCHAR(64) NOT NULL,
;        email VARCHAR(128) NOT NULL,
;        registered_at TIMESTAMP NOT NULL,
;        created_at TIMESTAMP,
;        updated_at TIMESTAMP,
;        UNIQUE (email)
;    )>)

If you need a ‘template’ for tables which doesn’t related to any database tables, you can use DAO-TABLE-MIXIN. Below the has-email class will not create a table.

(defclass has-email ()
  ((email :col-type (:varchar 128)
          :initarg :email
          :accessor object-email))
  (:metaclass mito:dao-table-mixin)
  (:unique-keys email))

(defclass user (has-email)
  ((name :col-type (:varchar 64)
         :initarg :name
         :accessor user-name))
  (:metaclass mito:dao-table-class))

(mito:table-definition 'user)
;       name VARCHAR(64) NOT NULL,
;       email VARCHAR(128) NOT NULL,
;       created_at TIMESTAMP,
;       updated_at TIMESTAMP,
;       UNIQUE (email)
;   )>)

See more examples of use in mito-auth.


“Cannot CHANGE-CLASS objects into CLASS metaobjects.”

If you get the following error message:

Cannot CHANGE-CLASS objects into CLASS metaobjects.
See also:
  The Art of the Metaobject Protocol, CLASS [:initialization]

it is certainly because you first wrote a class definition and then added the Mito metaclass and tried to evaluate the class definition again.

If this happens, you must remove the class definition from the current package:

(setf (find-class 'foo) nil)

or, with the Slime inspector, click on the class and find the “remove” button.

More info here.


First create the tables if needed:

(ensure-table-exists 'user)

then alter the tables, if needed:

(mito:migrate-table 'user)

You can check the SQL generated code with migration-expressions 'class. For example, we create the user table:

(ensure-table-exists 'user)
;       "name" VARCHAR(64) NOT NULL,
;       "email" VARCHAR(128),
;       "created_at" TIMESTAMP,
;       "updated_at" TIMESTAMP
;   ) () [0 rows] | MITO.DAO:ENSURE-TABLE-EXISTS

There are no changes from the previous user definition:

(mito:migration-expressions 'user)
;=> NIL

Now let’s add a unique email field:

(defclass user ()
  ((name :col-type (:varchar 64)
         :initarg :name
         :accessor user-name)
   (email :col-type (:varchar 128)
          :initarg :email
          :accessor user-email))
  (:metaclass mito:dao-table-class)
  (:unique-keys email))

The migration will run the following code:

(mito:migration-expressions 'user)
;=> (#<SXQL-STATEMENT: ALTER TABLE user ALTER COLUMN email TYPE character varying(128), ALTER COLUMN email SET NOT NULL>
;    #<SXQL-STATEMENT: CREATE UNIQUE INDEX unique_user_email ON user (email)>)

so let’s apply it:

(mito:migrate-table 'user)
;-> ;; ALTER TABLE "user" ALTER COLUMN "email" TYPE character varying(128), ALTER COLUMN "email" SET NOT NULL () [0 rows] | MITO.MIGRATION.TABLE:MIGRATE-TABLE
;   ;; CREATE UNIQUE INDEX "unique_user_email" ON "user" ("email") () [0 rows] | MITO.MIGRATION.TABLE:MIGRATE-TABLE
;-> (#<SXQL-STATEMENT: ALTER TABLE user ALTER COLUMN email TYPE character varying(128), ALTER COLUMN email SET NOT NULL>
;    #<SXQL-STATEMENT: CREATE UNIQUE INDEX unique_user_email ON user (email)>)


Creating objects

We can create user objects with the regular make-instance:

(defvar me
  (make-instance 'user :name "Eitaro Fukamachi" :email ""))
;=> USER

To save it in DB, use insert-dao:

(mito:insert-dao me)
;-> ;; INSERT INTO `user` (`name`, `email`, `created_at`, `updated_at`) VALUES (?, ?, ?, ?) ("Eitaro Fukamachi", "", "2016-02-04T19:55:16.365543Z", "2016-02-04T19:55:16.365543Z") [0 rows] | MITO.DAO:INSERT-DAO
;=> #<USER {10053C4453}>

Do the two steps above at once:

(mito:create-dao 'user :name "Eitaro Fukamachi" :email "")

You should not export the user class and create objects outside of its package (it is good practice anyway to keep all database-related operations in say a models package and file). You should instead use a helper function:

(defun make-user (&key name)
  (make-instance 'user :name name))

Updating fields

(setf (slot-value me 'name) "nitro_idiot")
;=> "nitro_idiot"

and save it:

(mito:save-dao me)


(mito:delete-dao me)
;-> ;; DELETE FROM `user` WHERE (`id` = ?) (1) [0 rows] | MITO.DAO:DELETE-DAO

;; or:
(mito:delete-by-values 'user :id 1)
;-> ;; DELETE FROM `user` WHERE (`id` = ?) (1) [0 rows] | MITO.DAO:DELETE-DAO

Get the primary key value

(mito:object-id me)
;=> 1


(mito:count-dao 'user)
;=> 1

Find one

(mito:find-dao 'user :id 1)
;-> ;; SELECT * FROM `user` WHERE (`id` = ?) LIMIT 1 (1) [1 row] | MITO.DB:RETRIEVE-BY-SQL
;=> #<USER {10077C6073}>

So here’s a possibility of generic helpers to find an object by a given key:

(defgeneric find-user (key-name key-value)
  (:documentation "Retrieves an user from the data base by one of the unique

(defmethod find-user ((key-name (eql :id)) (key-value integer))
  (mito:find-dao 'user key-value))

(defmethod find-user ((key-name (eql :name)) (key-value string))
  (first (mito:select-dao 'user
                          (sxql:where (:= :name key-value)))))

Find all

Use select-dao.

Get a list of all users:

(mito:select-dao 'user)
;=> (#<USER {10077C6073}>)

Find by relationship

As seen above:

(mito:find-dao 'tweet :user *user*)

Custom queries

It is with select-dao that you can write more precise queries by giving it SxQL statements.


(select-dao 'tweet
    (where (:like :status "%Japan%")))


See the SxQL documentation.


(select-dao 'foo
  (where (:and (:> :age 20) (:<= :age 65))))
(order-by :age (:desc :id))
(group-by :sex)
(having (:>= (:sum :hoge) 88))
(limit 0 10)

and joins, etc.


:is-null, :not-null
:asc, :desc
:=, :!=
:<, :>, :<= :>=
:a<, :a>
:in, :not-in
:and, :or
:+, :-, :* :/ :%


Since insert-dao, update-dao and delete-dao are defined as generic functions, you can define :before, :after or :around methods to those, like regular method combination.

(defmethod mito:insert-dao :before ((object user))
  (format t "~&Adding ~S...~%" (user-name object)))

(mito:create-dao 'user :name "Eitaro Fukamachi" :email "")
;-> Adding "Eitaro Fukamachi"...
;   ;; INSERT INTO "user" ("name", "email", "created_at", "updated_at") VALUES (?, ?, ?, ?) ("Eitaro Fukamachi", "", "2016-02-16 21:13:47", "2016-02-16 21:13:47") [0 rows] | MITO.DAO:INSERT-DAO
;=> #<USER {100835FB33}>


Inflation/Deflation is a function to convert values between Mito and RDBMS.

(defclass user-report ()
  ((title :col-type (:varchar 100)
          :initarg :title
          :accessor report-title)
   (body :col-type :text
         :initarg :body
         :initform ""
         :accessor report-body)
   (reported-at :col-type :timestamp
                :initarg :reported-at
                :initform (local-time:now)
                :accessor report-reported-at
                :inflate #'local-time:universal-to-timestamp
                :deflate #'local-time:timestamp-to-universal))
  (:metaclass mito:dao-table-class))

Eager loading

One of the pains in the neck to use ORMs is the “N+1 query” problem.


(use-package '(:mito :sxql))

(defvar *tweets-contain-japan*
  (select-dao 'tweet
    (where (:like :status "%Japan%"))))

;; Getting names of tweeted users.
(mapcar (lambda (tweet)
          (user-name (tweet-user tweet)))

This example sends a query to retrieve a user like “SELECT * FROM user WHERE id = ?” at each iteration.

To prevent this performance issue, add includes to the above query which only sends a single WHERE IN query instead of N queries:

;; GOOD EXAMPLE with eager loading

(use-package '(:mito :sxql))

(defvar *tweets-contain-japan*
  (select-dao 'tweet
    (includes 'user)
    (where (:like :status "%Japan%"))))
;-> ;; SELECT * FROM `tweet` WHERE (`status` LIKE ?) ("%Japan%") [3 row] | MITO.DB:RETRIEVE-BY-SQL
;-> ;; SELECT * FROM `user` WHERE (`id` IN (?, ?, ?)) (1, 3, 12) [3 row] | MITO.DB:RETRIEVE-BY-SQL
;=> (#<TWEET {1003513EC3}> #<TWEET {1007BABEF3}> #<TWEET {1007BB9D63}>)

;; No additional SQLs will be executed.
(tweet-user (first *))
;=> #<USER {100361E813}>

Schema versioning

$ ros install mito
$ mito
Usage: mito command [option...]


    -t, --type DRIVER-TYPE          DBI driver type (one of "mysql", "postgres" or "sqlite3")
    -d, --database DATABASE-NAME    Database name to use
    -u, --username USERNAME         Username for RDBMS
    -p, --password PASSWORD         Password for RDBMS
    -s, --system SYSTEM             ASDF system to load (several -s's allowed)
    -D, --directory DIRECTORY       Directory path to keep migration SQL files (default: "/Users/nitro_idiot/Programs/lib/mito/db/")
    --dry-run                       List SQL expressions to migrate


Mito provides some functions for introspection.

We can access the information of columns with the functions in (mito.class.column:...):

  • table-column-[class, name, info, not-null-p,...]
  • primary-key-p

and likewise for tables with (mito.class.table:...).

Given we get a list of slots of our class:

(ql:quickload "closer-mop")

(closer-mop:class-direct-slots (find-class 'user))

(defparameter user-slots *)

We can answer the following questions:

What is the type of this column ?

(mito.class.column:table-column-type (first user-slots))
;; (:VARCHAR 64)

Is this column nullable ?

  (first user-slots))
;; T
  (second user-slots))
;; NIL


We don’t want to test DB operations against the production one. We need to create a temporary DB before each test.

The macro below creates a temporary DB with a random name, creates the tables, runs the code and connects back to the original DB connection.

(defun random-string (length)
  ;; thanks 40ants/hacrm.
  (let ((chars "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"))
    (coerce (loop repeat length
                  collect (aref chars (random (length chars))))

(defmacro with-empty-db (&body body)
  "Run `body` with a new temporary DB.
  `(let* ((*random-state* (make-random-state t))
          (prefix (concatenate 'string
                               (random-string 8)
          (connection mito:*connection*))
     (uiop:with-temporary-file (:pathname name :prefix prefix)
       (let* ((*db-name* name)
              (*db* (connect)))
         (setf mito:*connection* connection)

Use it like this:

(prove:subtest "Creation in a temporary DB."
    (let ((user (make-user :name "Cookbook")))
      (save-user user)

      (prove:is (name user)
                "Test username in a temp DB."))))
;; Creation in a temporary DB
;;  CREATE TABLE "user" (
;;       name VARCHAR(64) NOT NULL,
;;       email VARCHAR(128) NOT NULL,
;;       created_at TIMESTAMP,
;;       updated_at TIMESTAMP,
;;       UNIQUE (email)
;; ) () [0 rows] | MITO.DB:EXECUTE-SQL
;; ✓ Test username in a temp DB.

See also