What is Jennifer?

Jennifer is an ActiveRecord-like ORM for Crystal with a custom DSL for queries and migrations.

This section is based on Jennifer Docs. Also see Amber Jennifer Example App.

Installing Jennifer for Amber

By default, Amber applications ship with Granite a lightweight ORM for Crystal applications. To begin using Jennifer in your Amber application you will need to follow a few steps to get up and running.

Generate a normal amber app

Begin by generating a new amber project (if you have not already).

amber new {project}
cd project

Update your project shard.yml

Once your Amber project is created, you will need to update your shard.yml file to include Jennifer as a dependency.

Add the following code snippet to your application's shard.yml file:

# Jennifer is an ORM
github: imdrasil/
# SAM is a CLI utility for running migrations, creating or dropping tables
github: imdrasil/

In addition to the Jennifer dependency, you may have noticed we included an additional shard called sam. The sam shard is simple command line utility for running common database tasks such as creating migrations, adding, or dropping tables. While technically optional it is recommended.

Once you've added the required dependencies, you will need to decide what database you would like to use with your Amber application. By default, Jennifer ships with SQLite support, however, should you want to use PostgreSQL or MySQL, you will need to add in a compatible database adapter to your shard.yml file.

# snipped shards
# Your choice of database adapter
# PostgreSQL
github: will/crystal-pg
version: "= 0.21.0"
github: crystal-lang/crystal-mysql
version: "= 0.11.0"

You can read about the Crystal community adapters below:

Once you have updated your dependencies and chosen an appropriate adapter (if any), proceed to update your project's shards by running:

shards update

This command will begin installing Jennifer and its associated dependencies into your Amber project.

Setup your database information

Once Jennifer is up an running as a dependency in your project, you will need to create a new database configuration. Inside of your project's config directory create a new file called config/database.yml and fill in the code snippet below:

defaults : &defaults
host: localhost
adapter: postgres
user: <add-your-database-user>
password: <add-your-database-password>
migration_files_path: db/migrations # this is the default location for all migrations
db: blog_development
<<: *defaults
db: blog_test
<<: *defaults

The config/database.yml describes the various databases Jennifer will use in your project's different environments.

Add A Jennifer Initializer to Configuration

With your config/database.yml in place, we need to inform Amber about Jennifer and tell the framework how we would like to configure the ORM. To begin, you will need to create a config/ file in your project's config directory. Below is a basic setup for Jennifer and you are also encourage to [see an example Amber Jennifer App] for more inspiration.

require "amber"
require "colorize""config/database.yml", Amber.env.to_s)
Jennifer::Config.configure do |conf|
conf.logger =
conf.logger.formatter = do |severity, datetime, progname, message, io|
io << datetime.colorize(:cyan) << ": \n" << message.colorize(:light_magenta)
conf.logger.level = Logger::DEBUG

Note that we pass the AMBER_ENV to this will allow Jennifer to use the correct database settings for the environment.

Create a in {project/src}

As mentioned previously, Jennifer uses Sam for running tasks pertinent to ORM operations. Sam is a Make-like utility which allows to specify tasks like Ruby's Rake do using plain Crystal. For how to use Sam visit the Github repository

Create a new file inside your project's src directory.

# src/
require "jennifer"
require "jennifer/adapter/postgres"
require "../config/jennifer"
require "../db/migrations/*"
require "sam"
require "jennifer/sam"
load_dependencies "jennifer"

This file operationalizes sam to begin running tasks. You can run crystal -- help to get a list of available tasks once the file is added.

Including Jennifer in Server Bootstrap

The final step in configuration is to include Jennifer in your src/{project}.cr file. This should be done before you load your application configurations (or at least models).

require "jennifer"
require "jennifer/adapter/postgres"
require "amber"
require "./controllers/**"
require "./mailers/**"
require "./models/**"
require "./views/**"
require "../config/*"

You're all set with the configuration. Next using Jennifer Migrations and Models.