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|
# frozen-string-literal: true
#
# The null_dataset extension adds the Dataset#nullify method, which
# returns a cloned dataset that will never issue a query to the
# database. It implements the null object pattern for datasets.
#
# The most common usage is probably in a method that must return
# a dataset, where the method knows the dataset shouldn't return
# anything. With standard Sequel, you'd probably just add a
# WHERE condition that is always false, but that still results
# in a query being sent to the database, and can be overridden
# using #unfiltered, the OR operator, or a UNION.
#
# Usage:
#
# ds = DB[:items].nullify.where(a: :b).select(:c)
# ds.sql # => "SELECT c FROM items WHERE (a = b)"
# ds.all # => [] # no query sent to the database
#
# Note that there is one case where a null dataset will sent
# a query to the database. If you call #columns on a nulled
# dataset and the dataset doesn't have an already cached
# version of the columns, it will create a new dataset with
# the same options to get the columns.
#
# This extension uses Object#extend at runtime, which can hurt performance.
#
# To add the nullify method to a single dataset:
#
# ds = ds.extension(:null_dataset)
#
# To add the nullify method to all datasets on a single database:
#
# DB.extension(:null_dataset)
#
# Related modules: Sequel::Dataset::Nullifiable, Sequel::Dataset::NullDataset
#
module Sequel
class Dataset
module Nullifiable
# Return a cloned nullified dataset.
def nullify
cached_dataset(:_nullify_ds) do
with_extend(NullDataset)
end
end
end
module NullDataset
# Create a new dataset from the dataset (which won't
# be nulled) to get the columns if they aren't already cached.
def columns
if cols = _columns
return cols
end
self.columns = db.dataset.clone(@opts).columns
end
# Return 0 without sending a database query.
def delete
0
end
# Return self without sending a database query, never yielding.
def each
self
end
# Return nil without sending a database query, never yielding.
def fetch_rows(sql)
nil
end
# Return nil without sending a database query.
def insert(*)
nil
end
# Return nil without sending a database query.
def truncate
nil
end
# Return 0 without sending a database query.
def update(v=OPTS)
0
end
protected
# Return nil without sending a database query.
def _import(columns, values, opts)
nil
end
private
# Just in case these are called directly by some internal code,
# make them noops. There's nothing we can do if the db
# is accessed directly to make a change, though.
(%w'_ddl _dui _insert' << '').each do |m|
class_eval("private; def execute#{m}(sql, opts=OPTS) end", __FILE__, __LINE__)
end
end
end
Dataset.register_extension(:null_dataset, Dataset::Nullifiable)
end
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