class Sequel::Dataset::PlaceholderLiteralizer

  1. lib/sequel/dataset/placeholder_literalizer.rb
  2. lib/sequel/dataset/prepared_statements.rb
  3. lib/sequel/dataset/query.rb
  4. lib/sequel/sql.rb
  5. show all
Superclass: Object

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.

Methods

Public Class

  1. loader
  2. new

Public Instance

  1. all
  2. each
  3. first
  4. freeze
  5. get
  6. sql
  7. with_dataset

Public Class methods

loader(dataset, &block)

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.

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
147 def self.loader(dataset, &block)
148   Recorder.new.loader(dataset, &block)
149 end
new(dataset, fragments, final_sql, arity)

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

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
152 def initialize(dataset, fragments, final_sql, arity)
153   @dataset = dataset
154   @fragments = fragments
155   @final_sql = final_sql
156   @arity = arity
157   freeze
158 end

Public Instance methods

all(*args, &block)

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.

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
179 def all(*args, &block)
180   @dataset.with_sql_all(sql(*args), &block)
181 end
each(*args, &block)

Run the SQL query for the given arguments, yielding each returned row to the block.

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
184 def each(*args, &block)
185   @dataset.with_sql_each(sql(*args), &block)
186 end
first(*args)

Run the SQL query for the given arguments, returning the first row.

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
189 def first(*args)
190   @dataset.with_sql_first(sql(*args))
191 end
freeze()

Freeze the fragments and final SQL when freezing the literalizer.

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
161 def freeze
162   @fragments.freeze
163   @final_sql.freeze
164   super
165 end
get(*args)

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).

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
195 def get(*args)
196   @dataset.with_sql_single_value(sql(*args))
197 end
sql(*args)

Return the SQL query to use for the given arguments.

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
200 def sql(*args)
201   raise Error, "wrong number of arguments (#{args.length} for #{@arity})" unless args.length == @arity
202   s = String.new
203   ds = @dataset
204   @fragments.each do |sql, i, transformer|
205     s << sql
206     if i.is_a?(Integer)
207       v = args.fetch(i)
208       v = transformer.call(v) if transformer
209     else
210       v = i.call
211     end
212     ds.literal_append(s, v)
213   end
214   s << @final_sql
215   s
216 end
with_dataset()

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.

[show source]
    # File lib/sequel/dataset/placeholder_literalizer.rb
170 def with_dataset
171   dataset = yield @dataset
172   other = dup
173   other.instance_variable_set(:@dataset, dataset)
174   other.freeze
175 end