Active Record
Active Record objects don’t specify their attributes directly, but rather
infer them from the table definition with which they’re linked. Adding,
removing, and changing attributes and their type is done directly in the
database. Any change is instantly reflected in the Active Record objects.
The mapping that binds a given Active Record class to a certain database
table will happen automatically in most common cases, but can be
overwritten for the uncommon ones.
See the mapping rules in table_name and the full example in
files/activerecord/README_rdoc.html
for more insight.
Creation
Active Records accept constructor parameters either in a hash or as a
block. The hash method is especially useful when you’re receiving the data
from somewhere else, like an HTTP request. It works like this:
user = User.new(:name => "David", :occupation => "Code Artist")
user.name # => "David"
You can also use block initialization:
user = User.new do |u|
u.name = "David"
u.occupation = "Code Artist"
end
And of course you can just create a bare object and specify the attributes
after the fact:
user = User.new
user.name = "David"
user.occupation = "Code Artist"
Conditions
Conditions can either be specified as a string, array, or hash representing
the WHERE-part of an SQL statement. The array form is to be used when the
condition input is tainted and requires sanitization. The string form can
be used for statements that don’t involve tainted data. The hash form works
much like the array form, except only equality and range is possible.
Examples:
class User < ActiveRecord::Base
def self.authenticate_unsafely(user_name, password)
where("user_name = '#{user_name}' AND password = '#{password}'").first
end
def self.authenticate_safely(user_name, password)
where("user_name = ? AND password = ?", user_name, password).first
end
def self.authenticate_safely_simply(user_name, password)
where(:user_name => user_name, :password => password).first
end
end
The authenticate_unsafely method inserts the parameters directly
into the query and is thus susceptible to SQL-injection attacks if the
user_name and password parameters come directly from an
HTTP request. The authenticate_safely and
authenticate_safely_simply both will sanitize the
user_name and password before inserting them in the
query, which will ensure that an attacker can’t escape the query and fake
the login (or worse).
When using multiple parameters in the conditions, it can easily become hard
to read exactly what the fourth or fifth question mark is supposed to
represent. In those cases, you can resort to named bind variables instead.
That’s done by replacing the question marks with symbols and supplying a
hash with values for the matching symbol keys:
Company.where(
"id = :id AND name = :name AND division = :division AND created_at > :accounting_date",
{ :id => 3, :name => "37signals", :division => "First", :accounting_date => '2005-01-01' }
).first
Similarly, a simple hash without a statement will generate conditions based
on equality with the SQL AND operator. For instance:
Student.where(:first_name => "Harvey", :status => 1)
Student.where(params[:student])
A range may be used in the hash to use the SQL BETWEEN operator:
Student.where(:grade => 9..12)
An array may be used in the hash to use the SQL IN operator:
Student.where(:grade => [9,11,12])
When joining tables, nested hashes or keys written in the form
‘table_name.column_name’ can be used to qualify the table name of a
particular condition. For instance:
Student.joins(:schools).where(:schools => { :type => 'public' })
Student.joins(:schools).where('schools.type' => 'public' )
Overwriting default accessors
All column values are automatically available through basic accessors on
the Active Record object, but sometimes you want to specialize this
behavior. This can be done by overwriting the default accessors (using the
same name as the attribute) and calling read_attribute(attr_name)
and write_attribute(attr_name, value) to actually change things.
class Song < ActiveRecord::Base
# Uses an integer of seconds to hold the length of the song
def length=(minutes)
write_attribute(:length, minutes.to_i * 60)
end
def length
read_attribute(:length) / 60
end
end
You can alternatively use self[:attribute]=(value) and
self[:attribute] instead of write_attribute(:attribute,
value) and read_attribute(:attribute).
Attribute query methods
In addition to the basic accessors, query methods are also automatically
available on the Active Record object. Query methods allow you to test
whether an attribute value is present.
For example, an Active Record User with the name attribute has a
name? method that you can call to determine whether the user has a
name:
user = User.new(:name => "David")
user.name? # => true
anonymous = User.new(:name => "")
anonymous.name? # => false
Accessing attributes before they have been typecasted
Sometimes you want to be able to read the raw attribute data without having
the column-determined typecast run its course first. That can be done by
using the <attribute>_before_type_cast accessors that all
attributes have. For example, if your Account model has a balance
attribute, you can call account.balance_before_type_cast or
account.id_before_type_cast.
This is especially useful in validation situations where the user might
supply a string for an integer field and you want to display the original
string back in an error message. Accessing the attribute normally would
typecast the string to 0, which isn’t what you want.
Dynamic attribute-based finders
Dynamic attribute-based finders are a cleaner way of getting (and/or
creating) objects by simple queries without turning to SQL. They work by
appending the name of an attribute to find_by_,
find_last_by_, or find_all_by_ and thus produces finders
like Person.find_by_user_name,
Person.find_all_by_last_name, and
Payment.find_by_transaction_id. Instead of writing
Person.where(:user_name => user_name).first, you just do
Person.find_by_user_name(user_name). And instead of writing
Person.where(:last_name => last_name).all, you just do
Person.find_all_by_last_name(last_name).
It’s also possible to use multiple attributes in the same find by
separating them with “and”.
Person.where(:user_name => user_name, :password => password).first
Person.find_by_user_name_and_password #with dynamic finder
Person.where(:user_name => user_name, :password => password, :gender => 'male').first
Payment.find_by_user_name_and_password_and_gender
It’s even possible to call these dynamic finder methods on relations and
named scopes.
Payment.order("created_on").find_all_by_amount(50)
Payment.pending.find_last_by_amount(100)
The same dynamic finder style can be used to create the object if it
doesn’t already exist. This dynamic finder is called with
find_or_create_by_ and will return the object if it already exists
and otherwise creates it, then returns it. Protected attributes won’t be
set unless they are given in a block.
# No 'Summer' tag exists
Tag.find_or_create_by_name("Summer") # equal to Tag.create(:name => "Summer")
# Now the 'Summer' tag does exist
Tag.find_or_create_by_name("Summer") # equal to Tag.find_by_name("Summer")
# Now 'Bob' exist and is an 'admin'
User.find_or_create_by_name('Bob', :age => 40) { |u| u.admin = true }
Use the find_or_initialize_by_ finder if you want to return a new
record without saving it first. Protected attributes won’t be set unless
they are given in a block.
# No 'Winter' tag exists
winter = Tag.find_or_initialize_by_name("Winter")
winter.new_record? # true
To find by a subset of the attributes to be used for instantiating a new
object, pass a hash instead of a list of parameters.
Tag.find_or_create_by_name(:name => "rails", :creator => current_user)
That will either find an existing tag named “rails”, or create a new one
while setting the user that created it.
Just like find_by_*, you can also use scoped_by_* to
retrieve data. The good thing about using this feature is that the very
first time result is returned using method_missing technique but
after that the method is declared on the class. Henceforth
method_missing will not be hit.
User.scoped_by_user_name('David')
Saving arrays, hashes, and other non-mappable objects in text columns
Active Record can serialize any object in text columns using YAML. To do
so, you must specify this with a call to the class method
serialize. This makes it possible to store arrays, hashes, and
other non-mappable objects without doing any additional work.
class User < ActiveRecord::Base
serialize :preferences
end
user = User.create(:preferences => { "background" => "black", "display" => large })
User.find(user.id).preferences # => { "background" => "black", "display" => large }
You can also specify a class option as the second parameter that’ll raise
an exception if a serialized object is retrieved as a descendant of a class
not in the hierarchy.
class User < ActiveRecord::Base
serialize :preferences, Hash
end
user = User.create(:preferences => %w( one two three ))
User.find(user.id).preferences # raises SerializationTypeMismatch
Single table inheritance
Active Record allows inheritance by storing the name of the class in a
column that by default is named “type” (can be changed by overwriting
Base.inheritance_column). This means that an inheritance looking
like this:
class Company < ActiveRecord::Base; end
class Firm < Company; end
class Client < Company; end
class PriorityClient < Client; end
When you do Firm.create(:name => "37signals"), this
record will be saved in the companies table with type = “Firm”. You can
then fetch this row again using Company.where(:name =>
'37signals').first and it will return a Firm object.
If you don’t have a type column defined in your table, single-table
inheritance won’t be triggered. In that case, it’ll work just like normal
subclasses with no special magic for differentiating between them or
reloading the right type with find.
Note, all the attributes for all the cases are kept in the same table. Read
more: www.martinfowler.com/eaaCatalog/singleTableInheritance.html
Connection to multiple databases in different models
Connections are usually created through ActiveRecord::Base.establish_connection
and retrieved by ActiveRecord::Base.connection. All
classes inheriting from ActiveRecord::Base will use
this connection. But you can also set a class-specific connection. For
example, if Course is an ActiveRecord::Base, but
resides in a different database, you can just say
Course.establish_connection and Course and all of its subclasses
will use this connection instead.
This feature is implemented by keeping a connection pool in ActiveRecord::Base that is a Hash indexed by the
class. If a connection is requested, the retrieve_connection
method will go up the class-hierarchy until a connection is found in the
connection pool.
Exceptions
-
ActiveRecordError - Generic error
class and superclass of all other errors raised by Active Record.
-
AdapterNotSpecified - The
configuration hash used in establish_connection didn't include an
:adapter key.
-
AdapterNotFound - The :adapter
key used in establish_connection specified a non-existent adapter
(or a bad spelling of an existing one).
-
AssociationTypeMismatch - The
object assigned to the association wasn't of the type specified in the
association definition.
-
SerializationTypeMismatch -
The serialized object wasn't of the class specified as the second
parameter.
-
ConnectionNotEstablished+ - No connection has been established. Use
establish_connection before querying.
-
RecordNotFound - No record responded to
the find method. Either the row with the given ID doesn't exist or
the row didn't meet the additional restrictions. Some find calls
do not raise this exception to signal nothing was found, please check its
documentation for further details.
-
StatementInvalid - The database server
rejected the SQL statement. The precise error is added in the message.
-
MultiparameterAssignmentErrors
- Collection of errors that occurred during a mass assignment using the
attributes= method. The errors property of this exception
contains an array of AttributeAssignmentError objects
that should be inspected to determine which attributes triggered the
errors.
-
AttributeAssignmentError - An
error occurred while doing a mass assignment through the
attributes= method. You can inspect the attribute
property of the exception object to determine which attribute triggered the
error.
Note: The attributes listed are class-level attributes (accessible
from both the class and instance level). So it's possible to assign a
logger to the class through Base.logger= which will then be used
by all instances in the current object space.