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

loader(dataset, &block) click to toggle source

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
new(dataset, fragments, final_sql, arity) click to toggle source

Save the dataset, array of SQL fragments, and ending SQL string.

    # File lib/sequel/dataset/placeholder_literalizer.rb
124 def initialize(dataset, fragments, final_sql, arity)
125   @dataset = dataset
126   @fragments = fragments
127   @final_sql = final_sql
128   @arity = arity
129   freeze
130 end

Public Instance Methods

all(*args, &block) click to toggle source

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
each(*args, &block) click to toggle source

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
first(*args) click to toggle source

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() click to toggle source

Freeze the fragments and final SQL when freezing the literalizer.

Calls superclass method
    # File lib/sequel/dataset/placeholder_literalizer.rb
133 def freeze
134   @fragments.freeze
135   @final_sql.freeze
136   super
137 end
get(*args) click to toggle source

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
sql(*args) click to toggle source

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
with_dataset() { |dataset;| ... } click to toggle source

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