Several model examples

class Contact < Jennifer::Model::Base
    id: Primary32, # same as {type: Int32, primary: true}
    name: String,
    gender: {type: String?, default: "male"},
    age: {type: Int32, default: 10},
    description: String?,
    created_at: Time?,
    updated_at: Time | Nil

  has_many :addresses, Address
  has_many :facebook_profiles, FacebookProfile
  has_and_belongs_to_many :countries, Country
  has_and_belongs_to_many :facebook_many_profiles, FacebookProfile, join_foreign: :profile_id
  has_one :main_address, Address, {where { _main }}
  has_one :passport, Passport

  validates_inclusion :age, 13..75
  validates_length :name, minimum: 1, maximum: 15
  validates_with_method :name_check

  scope :main { where { _age > 18 } }
  scope :older { |age| where { _age >= age } }
  scope :ordered { order(name: :asc) }

  def name_check
    if @description && @description.not_nil!.size > 10
      errors.add(:description, "Too large description")

class Address < Jennifer::Model::Base
    id: {type: Int32, primary: true},
    main: Bool,
    street: String,
    contact_id: Int32?,
    details: JSON::Any?
  validates_format :street, /st\.|street/

  belongs_to :contact, Contact

  scope :main { where { _main } }

class Passport < Jennifer::Model::Base
    enn: {type: String, primary: true},
    contact_id: {type: Int32, null: true}

  validates_with EnnValidator
  belongs_to :contact, Contact

class Profile < Jennifer::Model::Base
    id: Primary32,
    login: String,
    contact_id: Int32?,
    type: String

  belongs_to :contact, Contact

class FacebookProfile < Profile
    uid: String

  has_and_belongs_to_many :facebook_contacts, Contact, foreign: :profile_id

class TwitterProfile < Profile
    email: String

class Country < Jennifer::Model::Base
    id: Primary32,
    name: String

  validates_exclusion :name, ["asd", "qwe"]
  validates_uniqueness :name

  has_and_belongs_to_many :contacts, Contact

Mapping definition

You should define all fields that you’d like to grep from the particular table, other words - define model’s mapping.

.mapping(options, strict = true) macro stands for defining all model attributes. If field has no extra parameter, you can just specify name and type (type in case of crystal language): field_name: :Type. Named tuple can be used instead of type. Next keys are supported:

argument description
:type crystal data type
:primary mark field as primary key (default is false)
:null allows field to be nil (default is false for all fields except primary key
:default default value which will be set during creating new object
:column database column name associated with this attribute (default is attribute name)
:getter if getter should be created (default - true)
:setter if setter should be created (default - true)
:virtual mark field as virtual - will not be stored and retrieved from db
:converter class/module/object that is used to serialize/deserialize field
:auto indicate whether primary field is autoincrementable (by default true for Int32 and Int64)

Every .mapping call generates type alias AttrType which is a union of Jennifer::DBAny and any used arbitrary type.

To make some field nillable tou can use any of the next options:

  • pass null: true option to the named tuple
  • use ? in type declaration (e.g. some_field: String? or some_filed: {type: String?})
  • use union with Nil in the type declaration (e.g. some_field: String | Nil or some_filed: {type: String | Nil})

If you don’t want to define all the table fields - pass false as second argument (this will disable default strict mapping mode).

.mapping defines next methods:

method args description
.new Hash(String \| Symbol, DB::Any), NamedTuple, MySql::ResultSet constructors
.field_count   number of fields
.field_names   all fields names
._{{field_name}}   helper method for building queries
.coerce_{{field_name}} String coerces string to field_name type
.primary   returns criterion for primary field (query DSL)
.primary_field_name   name of primary field
.create Hash(String \| Symbol, DB::Any), NamedTuple creates object, stores it to db and returns it
.create! Hash(String \| Symbol, DB::Any), NamedTuple creates object, stores it to db and returns it; otherwise raise exception
.build Hash(String \| Symbol, DB::Any), NamedTuple builds object
.create Hash(String \| Symbol, DB::Any), NamedTuple builds object from hash and saves it to db with all callbacks
.create! Hash(String \| Symbol, DB::Any), NamedTuple builds object from hash and saves it to db with callbacks or raise exception
#{{field_name}}   getter
#{{field_name}}_changed?   presents whether field is changed
#{{field_name}}!   getter with not_nil! if null: true was passed
#{{field_name}}=   setter
#{{field_name}}_changed?   shows if field was changed
#new_record?   returns true if record has nil primary key (is not stored to db)
#changed?   shows if any field was changed
#primary   value of primary key field
#save   saves object to db; returns true if success and false elsewhere
#save!   saves object to db; returns true if success or rise exception otherwise
#to_h   returns hash with all attributes
#to_str_h   same as #to_h but with String keys
#attribute String \| Symbol returns attribute value by it’s name
#changed?   check if any field was changed
#set_attribute String \| Symbol, DB::Any sets attribute by given name
#attribute String \| Symbol returns attribute value by it’s name

Also #{{field_name}}? predicate method for the case when it is boolean.

All allowed types are listed on the Migration page.

All defined mapping properties are accessible via COLUMNS_METADATA constant and ::columns_tuple method.

It may be useful to have one parent class for all your models - just make it abstract and everything will work well:

abstract class ApplicationRecord < Jennifer::Model::Base

class SomeModel < ApplicationRecord
    id: Int32,
    name: String

Important restrictions:

  • models currently must have a primary field.
  • if your model also uses JSON.mapping, JSON::Serializable or other kinds of mapping macros, you must be careful to use Jennifer’s mapping macro last in order for all model features to work correctly.
class User < Jennifer::Model::Base
  # JSON.mapping used *before* model mapping:
  JSON.mapping(id: Int32, name: String)

  # Model mapping used last:
  mapping(id: Primary32, name: String)


To define a field converter create a class/module which implements next static methods:

  • .from_db(DB::ResultSet, NamedTuple) - converts field reading it from db result set;
  • .to_db(T, NamedTuple) - converts field to the db format;
  • .from_hash(Hash(String, Jennifer::DBAny | T), String, NamedTuple) - converts field (which name is the 2nd argument) from the given hash (this method is called only if hash has required key).

As an optional feature it also can implement .coerce(String, NamedTuple) method to be used for coercing. Doing this custom string parsing mechanism can be specified (for reference see Jennifer::Model::TimeZoneConverter).


This is default converter for Time field class - is applied automatically if other isn’t specified. It converts time object from UTC time to application time zone.

Next additional options can be specified in a field mapping:

  • time_zone_aware: Bool - disable time zone coverting for the field
  • time_format: String - custom time format that will be used to parse time-only strings (%H:%M by default)
  • date_format: String - custom date format that will be used to parse date-only string (%F by default)
  • date_time_format: String - custom date-time format that will be applied to parse date-time string (%F %T by default).

Also it is possible to customize how converter determines whether string is time/date/date-time. To do some you can inherit from Jennifer::Model::TimeZoneConverter converter and customize .time? and .date_time? methods.

Also you can manky-patch it


This is default converter for JSON::Any field class - is applied automatically if other isn’t specified. It takes care of JSON-string to JSON::Any conversion.


This converter allows to map enums to strings and back:

enum Category

class Note < Jennifer::Model::Base
    category: {type: Category?, converter: Jennifer::Model::EnumConverter(Category)}


Converts numeric database type to BigDecimal value which allows to perform operations with specific scale. It is expected that Float64 or PG::Numeric are used as an argument T.

It expects next options to be specified in a field mapping:

  • scale: Int32 - the count of decimal digits in the fractional part, to the right of decimal point.
class Order < Jennifer::Model::Base
    # ...
    total: { type: BigDecimal?, converter: Jennifer::Model::BigDecimalConverter(PG::Numeric), scale: 2 }
     # for MySQL use Float64


converts JSON to T class. T class should includes `JSON::Serializable.

class Location
  include JSON::Serializable

  property latitude : Float64
  property longitude : Float64

class User < Jennifer::Model::Base
    # ...
    location: { type: Location, converter: Jennifer::Model::JSONSerializableConverter(Location) }


Converts PG::Numeric to Float64. Can be used only when PostgreSQL adapter is used.


Converts ENUM database column type value to String. Can be used only when PostgreSQL adapter is used.

Arbitrary type

Model field can be of any type it is required to. But to achieve this you should specify corresponding converter to serialize/deserialize value to/from database format. One of the most popular examples is “embedded document” - JSON field that has known mapping and is mapped to crystal class.

For example see Jennifer::Model::JSONSerializableConverter(T) section above.

Now instances of Location class can be used in all constructors/setters/update methods. The only exception is query methods - they support only Jennifer::DBAny values.

Mapping Types

Jennifer has built-in system of predefined options for some usage. They are not data types on language level (you can’t defined variable of Primary32 type) and can be used only in mapping definition (standard usage).

class Post < Jennifer::Model::Base
    id: Primary32,
    # or even with full definition
    pk: {type: Primary32, primary: false, virtual: true}

All overrides from full definition will be respected and used instead of predefined for such a type.

To defined your own type define it such a way it may be lexically accessible from place you want to use it:

class ApplicationRecord < Jennifer::Model::Base
  EmptyString = {
    type:    String,
    default: "",

  {% TYPES << "EmptyString" %}
  # or if this is outside of model or view scope
  {% ::Jennifer::Macros::TYPES << "EmptyString" %}

Obviously, registered type added to the TYPES should be the same as defined constant; also it should be stringified.

Existing mapping types:

  • Primary32 = { type: Int32, primary: true }
  • Primary64 = { type: Int64, primary: true }
  • Password = { type: String?, virtual: true, setter: false }

Virtual attributes

If you pass virtual: true option for some field - it will not be stored to db and tried to be retrieved from there. Such behavior is useful if you have model-level attributes but it is not obvious to store them into db. Such approach allows mass assignment and dynamic get/set based on their name.

class User < Jennifer::Model::Base
    id: Primary32,
    password_hash: String,
    password: {type: String?, virtual: true},
    password_confirmation: {type: String?, virtual: true}

  validate_confirmation :password

  before_create :crypt_password

  def crypt_password
    self.password_hash =

User.create!(password: "qwe", password_confirmation: "qwe")

Table name

By default model determines related table name by underscoring and pluralizing own class name. In the case when model is define under some namespace, it’s underscored name is considered as table name prefix.

User.table_name # "users"
API::Admin::User.table_name # "api_admin_users"

To override table name prefix define own .table_prefix

module Admin
  class Base < Jennifer::Model::Base
    def self.table_prefix

  class User < Base
    mapping(id: Primary32)

Admin::User.table_name # "private_users"

As you see .table_prefix should return "_" at the end to keep naming across application consistent.

Also to prevent adding table prefix at all - return nil.

To override table name just call .table_name:

class User < Jennifer::Model::Base
  table_name :posts
  # ...

class Admin::User < Jennifer::Model::Base
  table_name "users"

User.table_name # "posts"
Admin::User.table_name # "users"

.table_name accepts table name that already includes prefix.

Converting form Web options

In the Web world all data got submitted forms will be recognized as Hash(String, String) which is not acceptable by your models. To resolve this use form_object shard.