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module ActiveRecord
module Validations
module ClassMethods
# Validates whether the value of the specified attributes are unique across the system. Useful for making sure that only one user
# can be named "davidhh".
#
# class Person < ActiveRecord::Base
# validates_uniqueness_of :user_name, :scope => :account_id
# end
#
# It can also validate whether the value of the specified attributes are unique based on multiple scope parameters. For example,
# making sure that a teacher can only be on the schedule once per semester for a particular class.
#
# class TeacherSchedule < ActiveRecord::Base
# validates_uniqueness_of :teacher_id, :scope => [:semester_id, :class_id]
# end
#
# When the record is created, a check is performed to make sure that no record exists in the database with the given value for the specified
# attribute (that maps to a column). When the record is updated, the same check is made but disregarding the record itself.
#
# Configuration options:
# * <tt>:message</tt> - Specifies a custom error message (default is: "has already been taken").
# * <tt>:scope</tt> - One or more columns by which to limit the scope of the uniqueness constraint.
# * <tt>:case_sensitive</tt> - Looks for an exact match. Ignored by non-text columns (+true+ by default).
# * <tt>:allow_nil</tt> - If set to true, skips this validation if the attribute is +nil+ (default is +false+).
# * <tt>:allow_blank</tt> - If set to true, skips this validation if the attribute is blank (default is +false+).
# * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
# occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>). The
# method, proc or string should return or evaluate to a true or false value.
# * <tt>:unless</tt> - Specifies a method, proc or string to call to determine if the validation should
# not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>). The
# method, proc or string should return or evaluate to a true or false value.
#
# === Concurrency and integrity
#
# Using this validation method in conjunction with ActiveRecord::Base#save
# does not guarantee the absence of duplicate record insertions, because
# uniqueness checks on the application level are inherently prone to race
# conditions. For example, suppose that two users try to post a Comment at
# the same time, and a Comment's title must be unique. At the database-level,
# the actions performed by these users could be interleaved in the following manner:
#
# User 1 | User 2
# ------------------------------------+--------------------------------------
# # User 1 checks whether there's |
# # already a comment with the title |
# # 'My Post'. This is not the case. |
# SELECT * FROM comments |
# WHERE title = 'My Post' |
# |
# | # User 2 does the same thing and also
# | # infers that his title is unique.
# | SELECT * FROM comments
# | WHERE title = 'My Post'
# |
# # User 1 inserts his comment. |
# INSERT INTO comments |
# (title, content) VALUES |
# ('My Post', 'hi!') |
# |
# | # User 2 does the same thing.
# | INSERT INTO comments
# | (title, content) VALUES
# | ('My Post', 'hello!')
# |
# | # ^^^^^^
# | # Boom! We now have a duplicate
# | # title!
#
# This could even happen if you use transactions with the 'serializable'
# isolation level. There are several ways to get around this problem:
# - By locking the database table before validating, and unlocking it after
# saving. However, table locking is very expensive, and thus not
# recommended.
# - By locking a lock file before validating, and unlocking it after saving.
# This does not work if you've scaled your Rails application across
# multiple web servers (because they cannot share lock files, or cannot
# do that efficiently), and thus not recommended.
# - Creating a unique index on the field, by using
# ActiveRecord::ConnectionAdapters::SchemaStatements#add_index. In the
# rare case that a race condition occurs, the database will guarantee
# the field's uniqueness.
#
# When the database catches such a duplicate insertion,
# ActiveRecord::Base#save will raise an ActiveRecord::StatementInvalid
# exception. You can either choose to let this error propagate (which
# will result in the default Rails exception page being shown), or you
# can catch it and restart the transaction (e.g. by telling the user
# that the title already exists, and asking him to re-enter the title).
# This technique is also known as optimistic concurrency control:
# http://en.wikipedia.org/wiki/Optimistic_concurrency_control
#
# Active Record currently provides no way to distinguish unique
# index constraint errors from other types of database errors, so you
# will have to parse the (database-specific) exception message to detect
# such a case.
def validates_uniqueness_of(*attr_names)
configuration = { :case_sensitive => true }
configuration.update(attr_names.extract_options!)
validates_each(attr_names,configuration) do |record, attr_name, value|
# The check for an existing value should be run from a class that
# isn't abstract. This means working down from the current class
# (self), to the first non-abstract class. Since classes don't know
# their subclasses, we have to build the hierarchy between self and
# the record's class.
class_hierarchy = [record.class]
while class_hierarchy.first != self
class_hierarchy.insert(0, class_hierarchy.first.superclass)
end
# Now we can work our way down the tree to the first non-abstract
# class (which has a database table to query from).
finder_class = class_hierarchy.detect { |klass| !klass.abstract_class? }
column = finder_class.columns_hash[attr_name.to_s]
if value.nil?
comparison_operator = "IS ?"
elsif column.text?
comparison_operator = "#{connection.case_sensitive_equality_operator} ?"
value = column.limit ? value.to_s.mb_chars[0, column.limit] : value.to_s
else
comparison_operator = "= ?"
end
sql_attribute = "#{record.class.quoted_table_name}.#{connection.quote_column_name(attr_name)}"
if value.nil? || (configuration[:case_sensitive] || !column.text?)
condition_sql = "#{sql_attribute} #{comparison_operator}"
condition_params = [value]
else
condition_sql = "LOWER(#{sql_attribute}) #{comparison_operator}"
condition_params = [value.mb_chars.downcase]
end
if scope = configuration[:scope]
Array(scope).map do |scope_item|
scope_value = record.send(scope_item)
condition_sql << " AND " << attribute_condition("#{record.class.quoted_table_name}.#{scope_item}", scope_value)
condition_params << scope_value
end
end
unless record.new_record?
condition_sql << " AND #{record.class.quoted_table_name}.#{record.class.primary_key} <> ?"
condition_params << record.send(:id)
end
finder_class.with_exclusive_scope do
if finder_class.exists?([condition_sql, *condition_params])
record.errors.add(attr_name, :taken, :default => configuration[:message], :value => value)
end
end
end
end
end
end
end
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