class Sequel::Dataset::PlaceholderLiteralizer
PlaceholderLiteralizer allows you to record the application of arbitrary changes to a dataset with placeholder arguments, recording where those placeholder arguments are used in the query. When running the query, the literalization process is much faster as Sequel can skip most of the work it normally has to do when literalizing a dataset.
Basically, this enables optimizations that allow Sequel to cache the SQL produced for a given dataset, so that it doesn't need to recompute that information every time.
Example:
loader = Sequel::Dataset::PlaceholderLiteralizer.loader(DB[:items]) do |pl, ds| ds.where(id: pl.arg).exclude(name: pl.arg).limit(1) end loader.first(1, "foo") # SELECT * FROM items WHERE ((id = 1) AND (name != 'foo')) LIMIT 1 loader.first(2, "bar") # SELECT * FROM items WHERE ((id = 2) AND (name != 'bar')) LIMIT 1
Caveats:
Note that this method does not handle all possible cases. For example:
loader = Sequel::Dataset::PlaceholderLiteralizer.loader(DB[:items]) do |pl, ds| ds.join(pl.arg, item_id: :id) end loader.all(:cart_items)
Will not qualify the item_id column with cart_items. In this type of situation it's best to add a table alias when joining:
loader = Sequel::Dataset::PlaceholderLiteralizer.loader(DB[:items]) do |pl, ds| ds.join(Sequel.as(pl.arg, :t), item_id: :id) end loader.all(:cart_items)
There are other similar cases that are not handled, mainly when Sequel changes the SQL produced depending on the types of the arguments.
Public Class Methods
Create a PlaceholderLiteralizer by yielding a Recorder and dataset to the given block, recording the offsets at which the recorders arguments are used in the query.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 146 def self.loader(dataset, &block) Recorder.new.loader(self, dataset, &block) end
Public Instance Methods
Return an array of all objects by running the SQL query for the given arguments. If a block is given, yields all objects to the block after loading them.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 178 def all(*args, &block) @dataset.with_sql_all(sql(*args), &block) end
Append the SQL query to use for the given arguments to the given SQL string.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 206 def append_sql(sql, *args) ds = @dataset @fragments.each do |s, i, transformer| sql << s if i.is_a?(Integer) v = args.fetch(i) v = transformer.call(v) if transformer else v = i.call end ds.literal_append(sql, v) end sql << @final_sql sql end
Run the SQL query for the given arguments, yielding each returned row to the block.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 183 def each(*args, &block) @dataset.with_sql_each(sql(*args), &block) end
Run the SQL query for the given arguments, returning the first row.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 188 def first(*args) @dataset.with_sql_first(sql(*args)) end
Freeze the fragments and final SQL when freezing the literalizer.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 160 def freeze @fragments.freeze @final_sql.freeze super end
Run the SQL query for the given arguments, returning the first value. For this to make sense, the dataset should return a single row with a single value (or no rows).
# File lib/sequel/dataset/placeholder_literalizer.rb, line 194 def get(*args) @dataset.with_sql_single_value(sql(*args)) end
Return the SQL query to use for the given arguments.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 199 def sql(*args) raise Error, "wrong number of arguments (#{args.length} for #{@arity})" unless args.length == @arity s = sql_origin append_sql(s, *args) end
Return a new PlaceholderLiteralizer with a modified dataset. This yields the receiver's dataset to the block, and the block should return the new dataset to use.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 169 def with_dataset dataset = yield @dataset other = dup other.instance_variable_set(:@dataset, dataset) other.freeze end
Private Instance Methods
# File lib/sequel/dataset/placeholder_literalizer.rb, line 224 def sql_origin String.new end