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, line 146
def self.loader(dataset, &block)
  Recorder.new.loader(self, 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 151
def initialize(dataset, fragments, final_sql, arity)
  @dataset = dataset
  @fragments = fragments
  @final_sql = final_sql
  @arity = arity
  freeze
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 178
def all(*args, &block)
  @dataset.with_sql_all(sql(*args), &block)
end
append_sql(sql, *args) click to toggle source

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
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 183
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 188
def first(*args)
  @dataset.with_sql_first(sql(*args))
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, line 160
def freeze
  @fragments.freeze
  @final_sql.freeze
  super
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 194
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 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
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 169
def with_dataset
  dataset = yield @dataset
  other = dup
  other.instance_variable_set(:@dataset, dataset)
  other.freeze
end

Private Instance Methods

sql_origin() click to toggle source
# File lib/sequel/dataset/placeholder_literalizer.rb, line 224
def sql_origin
  String.new
end