In this section, we'll provide a brief overview of how to get use out of Jennifer ORM. Note that this section is based on Jennifer Docs.

Jennifer in Action


Jennifer ORM uses a variety of macros to programatically define your model's associations and attributes. An important macro in particular is mapping which is used to describe model attributes. If a field has no extra parameter, you can specify name and type (type in case of crystal language): field_name: :Type. But you can use a tuple and provide the following parameters:



crystal data type (don't use question mark - for now you can use only :null option)


mark field as primary key (default is false)


allows field to be nil (default is false for all fields except primary key


default value which is set in creating a new object


if getter should be created (default - true)


if setter should be created (default - true)

Let's look at an example

class Contact < Jennifer::Model::Base
    id: {type: Int32, primary: true},
    name: String,
    gender: {type: String, default: "male", null: true},
    age: {type: Int32, default: 10},
    description: {type: String, null: true},
    created_at: {type: Time, null: true},
    updated_at: {type: Time, null: true}

In the above codeblock Jennifer's mapping macro is used to define a variety of attributes for the Contact model which map onto the database table neatly. Note that there are a variety of optional configurations such as default values and null security that can be appended to the mapping definition.

By default, expect that all fields are defined in the model. If that is not true, you should pass false as the second argument and override the ::field_count method to represent the correct field count.

Using a mapping will cause Jennifer to define the following methods on your model:



Hash(String | Symbol, DB::Any), NamedTuple, MySql::ResultSet



number of fields


all fields names




represents if field is changed


getter with not_nil! if null: true was passed




helper method for building queries


shows if field was changed


shows if any field was changed


value of primary key field


returns criteria for primary field (query dsl)


name of primary field


type of primary key


returns true if record has nil primary key (is not stored to db)


Hash(String | Symbol, DB::Any), NamedTuple

creates object, stores it to db and returns it


Hash(String | Symbol, DB::Any), NamedTuple

creates object, stores it to db and returns it; otherwise raise exception


Hash(String | Symbol, DB::Any), NamedTuple

builds object


Hash(String | Symbol, DB::Any), NamedTuple

builds object from hash and saves it to db with all callbacks


Hash(String | Symbol, DB::Any), NamedTuple

builds object from hash and saves it to db with callbacks or raise exception


saves object to db; returns true if success and false elsewhere


saves object to db; returns true if success or raise exception otherwise


returns hash with all attributes


same as #to_h but with String keys


String | Symbol

returns attribute value by its name


returns to_h with deleted nil entries


check if any field was changed


String | Symbol, DB::Any

sets attribute by given name


String | Symbol

returns attribute value by its name

The model is automatically associated with a table by its underscored and pluralized class name, but a custom name can be defined using the ::table_name method in its own body before using any relation (::singular_table_name - for singular variant).


Jennifer provides 4 types of relations out of the box: has_many, belongs_to, has_and_belongs_to_many, and has_one. While they sound similar they generate slightly different methods and behaviour.

class Contact < Jennifer::Model::Base
    id: {type: Int32, primary: true},
    name: String,
    gender: {type: String, default: "male", null: true},
    age: {type: Int32, default: 10},
    description: {type: String, null: true},
    created_at: {type: Time, null: true},
    updated_at: {type: Time, null: true}

  has_many :addresses, Address
  has_many :facebook_profiles, FacebookProfile

Relation take the following arguments:

  • Relation name

  • Target class

  • request - additional request (will be used inside of where clause) - optional

  • foreign - name of foreign key - optional; by default it uses the singularized table name + "_id"

  • primary - primary field name - optional; by default it uses the default primary field of the class.

has_and_belongs_to_many also accepts the next 2 arguments and uses regular arguments in a slightly different way:

  • join_table - join table name; by default, relation table names are in alphabetic order, joined by underscores.

  • join_foreign - foreign key for current model (left foreign key of join table)

  • foreign - used as right foreign key

  • primary - used as primary field of current table; for now it properly works only if both models in this relation have a primary field named id

All relation macros provide the following methods:

  • #{{relation_name}} - cache relation object (or array of them) and returns it.

  • #{{relation_name}}_reload - reload relation and returns it.

  • #{{relation_name}}_query - returns query which is used to get objects of this object relation entities from the db.

  • #remove_{{relation_name}} - removes given object from relation.

  • #add_{{relation_name}} - adds given object to relation or builds it from has and adds.

This allows for dynamically adding/removing objects to relations and automatically sets a foreign id:

contact = Contact.all.find!
contact.add_addresses({:main => true, :street => "some street", :details => nil})

address = contact.addresses.last

belongs_to and has_one add extra method #relation_name! which also adds assertion for nil.


Single table inheritance can be used in this way:

class Profile < Jennifer::Model::Base
    id: {type: Int32, primary: true},
    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

The subclass extends the superclass definition with new fields and uses string field type to identify itself.

Now Profile.all will return objects of Profile class, not taking into account the type field and will raise an exception if the superclass doesn't override ::field_count.

Scopes (aka. Prepared queries)

Scopes Jennifer's way of creating reusable prepared query statements. The scope macro allows you to define a Crystal block containing the desired prepared query statement, and creates a class method on the model to execute. The block is executed in the query context and provides a chainability with various Jennifer queries such as join, where, having, etc.

class Contact < Jennifer::Base::Model
	 scope :query_name { where { c("some_field") > 10 } }
	 scope :query_with_arguments { |a, b| where { (c("f1") == a) && (c("f2").in(b) } }

Scopes are helpful as they can be chained together to create complex queries which are readable and cognitively ergonomic.

ModelName.all.where { _some_field > 1 }
         .query_with_arguments("done", [1,2])
         .order(f1: :asc).no_argument_query


Jennifer uses accord for validations. Validations are mechanisms to run pre-flight checks on your models to ensure desired fields are present or values belong to a particular set. There are several general macros for declaring validations:

  • validates_with_method(*names) - accepts method name/names

  • validates_inclusion(field, value) - checks if value includes @{{field}}

  • validates_exclusion(field, value) - checks if value excludes @{{field}}

  • validates_format(field, format) - checks if {{format}} matches @{{field}}

  • validates_length(field, **options) - check @{{field}} size; allowed options are: :in, :is, :maximum, :minimum

  • validates_uniqueness(field) - check if @{{field}} is unique

If a record is not valid, it will not be saved.


Callbacks are Jennifer's way of providing hooks into the lifecycle of a record. The available macros for defining callbacks include:

  • before_save

  • after_save

  • before_create

  • after_create

  • after_initialize

  • before_destroy

They accept method names.


with_timestamps macros add callbacks for created_at and updated_at fields in an update.


To destroy an object, use .delete (called without callback) or .destroy. To destroy several objects by their ids, use the class method:

ids = [1, 20, 18]


There are several ways to update an object. Some of them were mentioned in the mapping section. A few extra methods to do this include:

  • #update_column(name, value) - sets attribute directly and stores it to the db without any callback.

  • #update_columns(values) - same for several columns.

  • #update_attributes(values) - just set attributes.

  • #set_attribute(name, value) - set attribute by given name.

Query DSL

Jennifer allows you to build lazy evaluated queries with a chainable syntax. However, some methods can only be used at the end of a chain (such as .first or .pluck). Here is a list of all DSL methods:


An object can be retrieved by id using find (returns T?) and find! (returns T or raise RecordNotFound exception) methods.



all retrieves everything (only at the beginning; creates empty request).


where clauses are extremely flexible. The where method accepts a block which represents the where clause of the query statement. You can chain several where methods together and they will be concatenated using AND.

To specify a field, use c method which accepts a string as the field name. As mentioned, after declaring model attributes, you can use their names inside of a block: _field_name if it is for the current table and ModelName._field_name if for another model. As a method name, you can specify an attribute of the model or table using underscores: _some_model_or_table_name__field_name - model/table name is separated from field name by "_". You can specify relation in space of which you want to declare condition using double at the beginning and block.

Below are a few examples:

Contact.where { c("id") == 1 }
Contact.where { _id == 1 }
Contact.all.join(Address) { Contact._id == _contact_id }
Contact.all.relation(:addresses).where { __addresses { _id > 1 } }

You can also use primary to mention primary field:

Passport.where {"%123%") }

Supported operators:

| Operator | SQL variant | | | --- | --- | | == | = | | | != | != | | | < | < | | | <= | <= | | | > | > | | | >= | >= | | | =~ | REGEXP, ~ | | | & | AND | | | ` | ` | OR |

And operator-like methods:

MethodSQL variant


REGEXP, ~ (accepts String)








IS and provided value


NOT and provided value (or as unary operator if no one is given)



And Postgres specific:

MethodSQL variant







Jennifer supports literal SQL query statements via the .sql method:

# it behaves like regular criteria
Contact.all.where { sql("age > ?",  [15]) & (_name == "Stephan") }

The query will be inserted "as is". Usage of .sql allows the use of nested plain request.


  • All regular expression methods accept string representation of the regular expression

  • Use parentheses for binary operators (& and |)

  • nil given to != and == will be transformed to IS NOT NULL and IS NULL

  • is and not operator accepts values: :nil, nil, :unknown, true, false

At the end - several examples:

Contact.where { (_id > 40) & _name.regexp("^[a-d]") }

Address.where { }


Raw SQL for SELECT clause can be passed into .select method. This has the highest priority in forming this query part."COUNT(id) as count,").group("name")
       .having { sql("COUNT(id)") > 1 }.pluck(:name)


Also you can provide a sub-query to specify the FROM clause.

Be careful when using sub-queries with source fields during result retrieving and mapping to objects.

Contact.all.from("select * from contacts where id > 2")
Contacts.all.from(Contact.where { _id > 2 })

Delete and Destroy

For now they both are the same - creates delete query with given conditions. destroy first loads objects and runs callbacks and then calls delete on each.

It can be only at the end of chain.

Address.where { _main.not }.delete


To join another table you can use join method passing model class or table name (String) and join type (default is :inner).

field = "contact_id"
table = "passports"
Contact.all.join(Address) { Contact._id == _contact_id }.join(table) { c(field) == _id }

Query, built inside of block, will be passed to ON section of JOIN. Current context of block is joined table.

Also there are two shortcuts for left and right joins:

Contact.all.left_join(Address) { _contacts__id == _contact_id }
Contact.all.right_join("addresses") { _contacts__id == c("contact_id") }

For now Jennifer provides manual aliasing as a second argument for #join and automatic when using #includes and #with methods. For details check out the code.


To join a model relation such as has_many, belongs_to and has_one you can pass it's name and join type as:

Contact.all.relation("addresses").relation(:passport, type: :left)


To preload a relation use includes and pass relation name:


If there are several includes with same table - Jennifer will auto alias tables.

Group"name", "id").pluck(:name, :id)

.group allows to add columns for GROUP BY section. If passing arguments are tuple of strings or just one string - all columns will be parsed as current table columns. If there is a need to group on joined table or using fields from several tables use next:

Contact.all.relation("addresses").group(addresses: ["street"], contacts: ["name"])
       .pluck("addresses.street", "")

Here keys should be table names.

Having"name").having { _age > 15 }

.having allows adding HAVING to the query. It accepts a block the same way as .where does.


Contact.where { _age > 42 }.exists? # returns true or false

.exists? check if there is any record with provided conditions. Can only be at the end of the query chain - it hits the db.


Contant.all.distinct("age") # returns array of ages (Array(DB::Any | Int16 | Int8))

.distinct retrieves database column values without repeats. It can accept column names and the table name as an optional second parameter. distinct can only be at the end of the query chain as it results in a direct database query.


There are 2 types of aggregation functions: ones which are working without a GROUP clause and returns single values (e.g. max, min, count), and ones working with a GROUP clause and returning an array of values.


Contact.all.max(:name, String)


Contact.all.min(:age, Int32)


Contact.all.avg(:age, Float64) # mysql specific
Contact.all.avg(:age, PG::Numeric) # Postgres specific


Contact.all.sum(:age, Float64) # mysql specific
Contact.all.sum(:age, Int64) # postgre specific



Group Max, Int32)

Group Min, Int32)

Group Avg

Contact.all.avg(:age, Float64) # mysql specific
Contact.all.avg(:age, PG::Numeric) # Postgres specific

Group Sum, Float64) # mysql specific, Int64) # postgre specific


For now you can only specify limit and offset:



You can specifies orders to sort:

Contact.all.order(name: :asc, id: "desc")

It accepts hash as well.


You can provide a hash or a named tuple with new field values:

Contact.all.update(age: 1, name: "Wonder")

Will not trigers any callback.

Eager load

As mentioned previously, Jennifer provides lazy query evaluation, this means it will perform the operation only after trying to access an element from the collection (any array method - it implements Enumerable). Also you can extract the first entity via first. If you are sure that at least one entity in the database satisfies your query, you can call .first!.

To extract only some fields rather than entire objects, use pluck:

Contact.all.pluck(:id, "name")

It returns an array of values if only one field was given, and an array of arrays if more. It accepts raw sql arguments, so be carefuo when using this when joining tables with same field names. This allows to retrieving custom data from specified select clauses."COUNT(id) as count,").group("name")
       .having { sql("COUNT(id)") > 1 }.pluck(:count)

To load relations using same query joins needed tables (yep you should specify join on condition by yourself again) and specifies all needed relations in with (relation name not table).

Contact.all.left_join(Address) { _contacts__id == _contact_id }.with(:addresses)


Transaction mechanism provides block-like syntax:

Jennifer::Adapter.adapter.transaction do |tx|
  Contact.create({:name => "Chose", :age => 20})

If an error was raised in the transaction block, the transaction will be rolled back. To rollback a transaction, raise DB::Rollback exception.

A transaction will lock the connection for the current fiber, avoiding getting a new one from pool.


To truncate an entire table use:

# or

This functionality could be useful to clear the db between test cases.

Important restrictions

  • sqlite3 has a lot of limitations so its support will not be added soon.


The quickest way to rollback any changes made to the database after a test case is using transaction. You can ensure smooth testing by adding the following to your

Spec.before_each do

Spec.after_each do

Also just regular deleting or truncation could be used but transactions provide 15x speed up (at least for postgres; mysql gets less impact).

This functions can be safely used only under test environment.

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