Class: Sequel::Dataset::PlaceholderLiteralizer

Inherits:
Object
  • Object
show all
Defined in:
lib/sequel/dataset/placeholder_literalizer.rb

Overview

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.

Defined Under Namespace

Classes: Argument, Recorder

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(dataset, fragments, final_sql, arity) ⇒ PlaceholderLiteralizer

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


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 124

def initialize(dataset, fragments, final_sql, arity)
  @dataset = dataset
  @fragments = fragments
  @final_sql = final_sql
  @arity = arity
  freeze
end

Class Method Details

.loader(dataset, &block) ⇒ Object

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.


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 119

def self.loader(dataset, &block)
  Recorder.new.loader(dataset, &block)
end

Instance Method Details

#all(*args, &block) ⇒ Object

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.


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 148

def all(*args, &block)
  @dataset.with_sql_all(sql(*args), &block)
end

#each(*args, &block) ⇒ Object

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


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 153

def each(*args, &block)
  @dataset.with_sql_each(sql(*args), &block)
end

#first(*args) ⇒ Object

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


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 158

def first(*args)
  @dataset.with_sql_first(sql(*args))
end

#freezeObject

Freeze the fragments and final SQL when freezing the literalizer.


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 133

def freeze
  @fragments.freeze
  @final_sql.freeze
  super
end

#get(*args) ⇒ Object

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).


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 164

def get(*args)
  @dataset.with_sql_single_value(sql(*args))
end

#sql(*args) ⇒ Object

Return the SQL query to use for the given arguments.

Raises:


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 169

def sql(*args)
  raise Error, "wrong number of arguments (#{args.length} for #{@arity})" unless args.length == @arity
  s = String.new
  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_datasetObject

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.


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# File 'lib/sequel/dataset/placeholder_literalizer.rb', line 142

def with_dataset
  dup.instance_exec{@dataset = yield @dataset; self}.freeze
end