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module ActiveRecord
module AttributeMethods
module Read
extend ActiveSupport::Concern
ATTRIBUTE_TYPES_CACHED_BY_DEFAULT = [:datetime, :timestamp, :time, :date]
included do
class_attribute :attribute_types_cached_by_default, instance_writer: false
self.attribute_types_cached_by_default = ATTRIBUTE_TYPES_CACHED_BY_DEFAULT
end
module ClassMethods
# +cache_attributes+ allows you to declare which converted attribute
# values should be cached. Usually caching only pays off for attributes
# with expensive conversion methods, like time related columns (e.g.
# +created_at+, +updated_at+).
def cache_attributes(*attribute_names)
cached_attributes.merge attribute_names.map { |attr| attr.to_s }
end
# Returns the attributes which are cached. By default time related columns
# with datatype <tt>:datetime, :timestamp, :time, :date</tt> are cached.
def cached_attributes
@cached_attributes ||= columns.select { |c| cacheable_column?(c) }.map { |col| col.name }.to_set
end
# Returns +true+ if the provided attribute is being cached.
def cache_attribute?(attr_name)
cached_attributes.include?(attr_name)
end
protected
# We want to generate the methods via module_eval rather than
# define_method, because define_method is slower on dispatch.
# Evaluating many similar methods may use more memory as the instruction
# sequences are duplicated and cached (in MRI). define_method may
# be slower on dispatch, but if you're careful about the closure
# created, then define_method will consume much less memory.
#
# But sometimes the database might return columns with
# characters that are not allowed in normal method names (like
# 'my_column(omg)'. So to work around this we first define with
# the __temp__ identifier, and then use alias method to rename
# it to what we want.
#
# We are also defining a constant to hold the frozen string of
# the attribute name. Using a constant means that we do not have
# to allocate an object on each call to the attribute method.
# Making it frozen means that it doesn't get duped when used to
# key the @attributes_cache in read_attribute.
def define_method_attribute(name)
safe_name = name.unpack('h*').first
temp_method = "__temp__#{safe_name}"
ActiveRecord::AttributeMethods::AttrNames.set_name_cache safe_name, name
generated_attribute_methods.module_eval <<-STR, __FILE__, __LINE__ + 1
def #{temp_method}
name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{safe_name}
read_attribute(name) { |n| missing_attribute(n, caller) }
end
STR
generated_attribute_methods.module_eval do
alias_method name, temp_method
undef_method temp_method
end
end
private
def cacheable_column?(column)
if attribute_types_cached_by_default == ATTRIBUTE_TYPES_CACHED_BY_DEFAULT
! serialized_attributes.include? column.name
else
attribute_types_cached_by_default.include?(column.type)
end
end
end
# Returns the value of the attribute identified by <tt>attr_name</tt> after
# it has been typecast (for example, "2004-12-12" in a data column is cast
# to a date object, like Date.new(2004, 12, 12)).
def read_attribute(attr_name)
# If it's cached, just return it
# We use #[] first as a perf optimization for non-nil values. See https://gist.github.com/jonleighton/3552829.
name = attr_name.to_s
@attributes_cache[name] || @attributes_cache.fetch(name) {
column = @columns_hash.fetch(name) {
return @attributes.fetch(name) {
if name == 'id' && self.class.primary_key != name
read_attribute(self.class.primary_key)
end
}
}
value = @attributes.fetch(name) {
return block_given? ? yield(name) : nil
}
if self.class.cache_attribute?(name)
@attributes_cache[name] = column.type_cast(value)
else
column.type_cast value
end
}
end
private
def attribute(attribute_name)
read_attribute(attribute_name)
end
end
end
end
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