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# frozen-string-literal: true
module Sequel
module Plugins
# This plugin implements optimistic locking mechanism on PostgreSQL based
# on the xmin of the row. The xmin system column is automatically set to
# the current transaction id whenever the row is inserted or updated:
#
# class Person < Sequel::Model
# plugin :pg_xmin_optimistic_locking
# end
# p1 = Person[1]
# p2 = Person[1]
# p1.update(name: 'Jim') # works
# p2.update(name: 'Bob') # raises Sequel::NoExistingObject
#
# The advantage of pg_xmin_optimistic_locking plugin compared to the
# regular optimistic_locking plugin as that it does not require any
# additional columns setup on the model. This allows it to be loaded
# in the base model and have all subclasses automatically use
# optimistic locking. The disadvantage is that testing can be
# more difficult if you are modifying the underlying row between
# when a model is retrieved and when it is saved.
#
# This plugin may not work with the class_table_inheritance plugin.
#
# This plugin relies on the instance_filters plugin.
module PgXminOptimisticLocking
WILDCARD = LiteralString.new('*').freeze
# Define the xmin column accessor
def self.apply(model)
model.instance_exec do
plugin(:optimistic_locking_base)
@lock_column = :xmin
def_column_accessor(:xmin)
end
end
# Update the dataset to append the xmin column if it is usable
# and there is a dataset for the model.
def self.configure(model)
model.instance_exec do
set_dataset(@dataset) if @dataset
end
end
module ClassMethods
private
# Ensure the dataset selects the xmin column if doing so
def convert_input_dataset(ds)
append_xmin_column_if_usable(super)
end
# If the xmin column is not already selected, and selecting it does not
# raise an error, append it to the selections.
def append_xmin_column_if_usable(ds)
select = ds.opts[:select]
unless select && select.include?(:xmin)
xmin_ds = ds.select_append(:xmin)
begin
columns = xmin_ds.columns!
rescue Sequel::DatabaseConnectionError, Sequel::DatabaseDisconnectError
raise
rescue Sequel::DatabaseError
# ignore, could be view, subquery, table returning function, etc.
else
ds = xmin_ds if columns.include?(:xmin)
end
end
ds
end
end
module InstanceMethods
private
# Only set the lock column instance filter if there is an xmin value.
def lock_column_instance_filter
super if @values[:xmin]
end
# Include xmin value when inserting initial row
def _insert_dataset
super.returning(WILDCARD, :xmin)
end
# Remove the xmin from the columns to update.
# PostgreSQL automatically updates the xmin value, and it cannot be assigned.
def _save_update_all_columns_hash
v = super
v.delete(:xmin)
v
end
# Add an RETURNING clause to fetch the updated xmin when updating the row.
def _update_without_checking(columns)
ds = _update_dataset
rows = ds.clone(ds.send(:default_server_opts, :sql=>ds.returning(:xmin).update_sql(columns))).all
values[:xmin] = rows.first[:xmin] unless rows.empty?
rows.length
end
end
end
end
end
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