Model Code Snippets

def Order < ActiveRecord::Base
 has_many :line_items
 belongs_to :customer   # there's a column "customer_id" in the db table
end

def LineItem < ActiveRecord::Base
 belongs_to :order # there's a column "order_id" in the db table
end

def Customer < ActiveRecord::Base
 has_many :orders
 has_one :address
end

def Address < ActiveRecord::Base
 belongs_to :customer
end

belongs_to  :some_model,
       :class_name  => 'MyClass',      # specifies other class name
       :foreign_key => 'my_real_id',   # and primary key
       :conditions  => 'column = 0'    # only finds when this condition met

has_one :some_model,
       # as belongs_to and additionally:
       :dependent   => :destroy        # deletes associated object
       :order       => 'name ASC'      # SQL fragment for sorting

has_many :some_model
       # as has_one and additionally:
       :dependent => :destroy          # deletes all dependent data
                                       # calling each objects destroy
       :dependent => :delete_all       # deletes all dependent data
                                       # without calling the destroy methods
       :dependent => :nullify          # set association to null, not
                                       # destroying objects
       :group => 'name'                # adds GROUP BY fragment
       :finder_sql => 'select ....'    # instead of the Rails finders
       :counter_sql => 'select ...'    # instead of the Rails counters


validates_presence_of :firstname, :lastname     # must be filled out

validates_length_of :password,
                   :minimum => 8           # more than 8 characters
                   :maximum => 16          # shorter than 16 characters
                   :in => 8..16            # between 8 and 16 characters
                   :too_short => 'way too short'   
                   :too_long => 'way to long'

validates_acceptance_of :eula               # Must accept a condition
                       :accept => 'Y'      # default: 1 (ideal for a checkbox)

validates_confirmation_of :password    
# the fields password and password_confirmation must match

validates_uniqueness_of :user_name              # user_name has to be unique
                       :scope => 'account_id'  # Condition:
                                               # account_id = user.account_id

validates_format_of :email          # field must match a regular expression
                   :with => /^(+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i

validates_numericality_of   :value                  # value is numeric
                           :only_integer => true  
                           :allow_nil => true     

validates_inclusion_of  :gender,    # value is in enumeration
                       :in => %w( m, f )

validates_exclusion_of  :age            # value is not in Enumeration
                       :in => 13..19   # don't want any teenagers

validates_associated :relation 
# validates that the associated object is valid

Options for all validations above:
:message => 'my own errormessage'   # eigene Fehlermeldung
:on      => :create                 # or :update (validates only then)
:if      => ...                     # call method oder Proc