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 normal 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(: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(: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, line 116
def self.loader(dataset, &block)
  Recorder.new.loader(dataset, &block)
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, line 121
def initialize(dataset, fragments, final_sql, arity)
  @dataset = dataset
  @fragments = fragments
  @final_sql = final_sql
  @arity = arity
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, line 137
def all(*args, &block)
  @dataset.with_sql_all(sql(*args), &block)
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, line 142
def each(*args, &block)
  @dataset.with_sql_each(sql(*args), &block)
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, line 147
def first(*args)
  @dataset.with_sql_first(sql(*args))
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, line 153
def get(*args)
  @dataset.with_sql_single_value(sql(*args))
end
sql(*args) click to toggle source

Return the SQL query to use for the given arguments.

# File lib/sequel/dataset/placeholder_literalizer.rb, line 158
def sql(*args)
  raise Error, "wrong number of arguments (#{args.length} for #{@arity})" unless args.length == @arity
  s = ''
  ds = @dataset
  @fragments.each do |sql, i, transformer|
    s << sql
    if i.is_a?(Integer)
      v = args.fetch(i)
      v = transformer.call(v) if transformer
    else
      v = i.call
    end
    ds.literal_append(s, v)
  end
  if sql = @final_sql
    s << sql
  end
  s
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, line 131
def with_dataset
  dup.instance_exec{@dataset = yield @dataset; self}
end