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Guides - Entities |
An entity is model domain object that is defined by its identity. See "Domain Driven Design" by Eric Evans.
An entity is at the core of an application, where the part of the domain logic is implemented. It's a small, cohesive object that expresses coherent and meaningful behaviors.
It deals with one and only one responsibility that is pertinent to the domain of the application, without caring about details such as persistence or validations.
This simplicity of design allows developers to focus on behaviors, or message passing if you will, which is the quintessence of Object Oriented Programming.
Internally, an entity holds a schema of the attributes, made of their names and types. The role of a schema is to whitelist the data used during the initialization, and to enforce data integrity via coercions or exceptions.
We'll see concrete examples in a second.
When using a SQL database, this is derived automatically from the table definition.
Imagine to have the books
table defined as:
CREATE TABLE `books` (
`id` integer NOT NULL PRIMARY KEY AUTOINCREMENT,
`title` varchar(255),
`created_at` timestamp,
`updated_at` timestamp
);
This is the corresponding entity Book
.
# lib/bookshelf/entities/book.rb
class Book < Hanami::Entity
end
Let's instantiate it with proper values:
book = Book.new(title: "Hanami")
book.title # => "Hanami"
book.created_at # => nil
The created_at
attribute is nil
because it wasn't present when we have instantiated book
.
It ignores unknown attributes:
book = Book.new(unknown: "value")
book.unknown # => NoMethodError
book.foo # => NoMethodError
It raises a NoMethodError
both for unknown
and foo
, because they aren't part of the internal schema.
It can coerce values:
book = Book.new(created_at: "Sun, 13 Nov 2016 09:41:09 GMT")
book.created_at # => 2016-11-13 09:41:09 UTC
book.class # => Time
An entity tries as much as it cans to coerce values according to the internal schema.
It enforces data integrity via exceptions:
Book.new(created_at: "foo") # => ArgumentError
If we use this feature, in combination with database constraints and validations, we can guarantee a strong level of data integrity for our projects.
We can take data integrity a step further: we can optionally define our own entity internal schema.
Custom schema is optional for SQL databases, while it's mandatory for entities without a database table, or while using with a non-SQL database.
# lib/bookshelf/entities/user.rb
class User < Hanami::Entity
EMAIL_FORMAT = /\@/
attributes do
attribute :id, Types::Int
attribute :name, Types::String
attribute :email, Types::String.constrained(format: EMAIL_FORMAT)
attribute :age, Types::Int.constrained(gt: 18)
attribute :codes, Types::Collection(Types::Coercible::Int)
attribute :comments, Types::Collection(Comment)
attribute :created_at, Types::Time
attribute :updated_at, Types::Time
end
end
Let's instantiate it with proper values:
user = User.new(name: "Luca", age: 34, email: "test@hanami.test")
user.name # => "Luca"
user.age # => 34
user.email # => "luca@hanami.test"
user.codes # => nil
user.comments # => nil
It can coerce values:
user = User.new(codes: ["123", "456"])
user.codes # => [123, 456]
Other entities can be passed as concrete instance:
user = User.new(comments: [Comment.new(text: "cool")])
user.comments
# => [#<Comment:0x007f966be20c58 @attributes={:text=>"cool"}>]
Or as data:
user = User.new(comments: [{text: "cool"}])
user.comments
# => [#<Comment:0x007f966b689e40 @attributes={:text=>"cool"}>]
It enforces data integrity via exceptions:
User.new(email: "foo") # => TypeError: "foo" (String) has invalid type for :email
User.new(comments: [:foo]) # => TypeError: :foo must be coercible into Comment
Custom schema takes precedence over automatic schema. If we use custom schema, we're need to add manually all the new columns from the corresponding SQL database table.
Learn more about data types in the dedicated article.