Models in Merb

You can find a quick overview of DataMapper features below, but you should check out the DataMapper documentation for more in-depth information.

When defining a model in Datamapper you specify the properties in the model file. This is handy when you want to see what the column was that has all of your headline content in it. The basic syntax is this…

property :field_name, String # String is the class of the variable type you would like to use

Class names that you can use for variable types are…

• TrueClass, Boolean
• String
• Text
• Float
• Fixnum, Integer
• BigDecimal
• DateTime, Date, Time

Datamapper also provides more complex data-types if you include DM-Types …

• Csv
• Enum
• EpochTime
• FilePath
• Flag
• IPAddress
• URI
• Yaml
• Json
• BCryptHash
• Regexp (incorrectly called Regex in the DataMapper docs)

You can assign various options to your properties, most of them should look pretty familiar.

:public, :protected, :private, :accessor, :reader, :writer,
:lazy, :default, :nullable, :key, :serial, :field, :size, :length,
:format, :index, :check, :ordinal, :auto_validation, :validates, :unique,
:lock, :track, :scale, :precision

:key          - Set as primary key. Can't use :key as a field name.
:serial       - auto-incrementing key
:lazy         - Lazy load the specified property (:lazy => true).
:default      - Specifies the default value
:field        - Specifies the table column
:nullable     - Can the value be null?
:index        - Creates a database index for the column
:accessor     - Set method visibility for the property accessors. Affects both
                reader and writer. Allowable values are :public, :protected, :private.
:reader       - Like the accessor option but affects only the property reader.
:writer       - Like the accessor option but affects only the property writer.
:protected    - Alias for :reader => :public, :writer => :protected
:private      - Alias for :reader => :public, :writer => :private

There are only a few subtle differences in datamapper models compared to active record models. has_many in Merb uses 'has' and then the association you would like, see below.

has n, :pugs # has many
has 1, :pug # has exactly 1
has n..n # has from n to n. e.g. - 1..3, 0..2, etc.
belongs_to :owner

Possible options are :through, :child_key, :class

Here are a few examples of the model property validation methods provided by DataMapper. Add your own examples below.

 
# ensure #name is always set
validates_present :name
validates_present :name, :with => [:create, :update]
 
# ensure #spam is never set
validates_absent :spam
 
# check the length of #zip_code is exactly 5 characters
validates_length :zip_code, :is => 5 
 
# check the length of #age is between 1 and 3 characters
validates_length :age, :in => 1..3 
 
# check that #email and #email_confirmation have the same value
validates_is_confirmed :email
 
# check that #email and #email_repeat have the same value
validates_is_confirmed :email, :confirm => :email_repeat
 
# check that #registration matches the given regex
validates_format :registration, :with => /[A-Z]{3}-[0-9]{3}/
 
# check that #email is a valid email address
validates_format :email, :as => :email_address
 
# check that #homepage is a valid URL
validates_format :homepage, :as => :url
 
# check that #age is an integer
validates_is_number :age, :integer_only => true
 
# check that #percentage is a fixnum
validates_is_number :percentage
 
# ensure #email is unique
validates_is_unique :email
 
# execute an arbitrary check against #age
validates_with_method :age, :less_than_130?
 
# execute an arbitrary check against #age
validates_with_block :age do
  return [false, "I doubt you're over 130."] if @age >= 130
  true
end

TODO Restructure this section to provide more explanation and better examples for each validation method.

Once you are happy with your models or anytime you need to update them you need to migrate/upgrade your database. With Merb and Datamapper there are 2 ways to do this. One is destructive(automigrate) and the other is not(autoupgrade). To see more information about these just run 'rake -T' in one of your apps and look for the rake tasks that start with 'db'

# Destructive migrations
# Recreates your db, erasing all data

rake db:automigrate

# Friendly migrations
# upgrades your table without destroying any data.
rake db:autoupgrade