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 119 def self.loader(dataset, &block) 120 Recorder.new.loader(dataset, &block) 121 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 148 def all(*args, &block) 149 @dataset.with_sql_all(sql(*args), &block) 150 end
Run the SQL
query for the given arguments, yielding each returned row to the block.
# File lib/sequel/dataset/placeholder_literalizer.rb 153 def each(*args, &block) 154 @dataset.with_sql_each(sql(*args), &block) 155 end
Run the SQL
query for the given arguments, returning the first row.
# File lib/sequel/dataset/placeholder_literalizer.rb 158 def first(*args) 159 @dataset.with_sql_first(sql(*args)) 160 end
Freeze the fragments and final SQL
when freezing the literalizer.
# File lib/sequel/dataset/placeholder_literalizer.rb 133 def freeze 134 @fragments.freeze 135 @final_sql.freeze 136 super 137 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 164 def get(*args) 165 @dataset.with_sql_single_value(sql(*args)) 166 end
Return the SQL
query to use for the given arguments.
# File lib/sequel/dataset/placeholder_literalizer.rb 169 def sql(*args) 170 raise Error, "wrong number of arguments (#{args.length} for #{@arity})" unless args.length == @arity 171 s = String.new 172 ds = @dataset 173 @fragments.each do |sql, i, transformer| 174 s << sql 175 if i.is_a?(Integer) 176 v = args.fetch(i) 177 v = transformer.call(v) if transformer 178 else 179 v = i.call 180 end 181 ds.literal_append(s, v) 182 end 183 if sql = @final_sql 184 s << sql 185 end 186 s 187 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 142 def with_dataset 143 dup.instance_exec{@dataset = yield @dataset; self}.freeze 144 end