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.
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
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 130 def all(*args, &block) @dataset.with_sql_all(sql(*args), &block) end
Run the SQL query for the given arguments, yielding each returned row to the block.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 135 def each(*args, &block) @dataset.with_sql_each(sql(*args), &block) end
Run the SQL query for the given arguments, returning the first row.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 140 def first(*args) @dataset.with_sql_first(sql(*args)) end
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 146 def get(*args) @dataset.with_sql_single_value(sql(*args)) end
Return the SQL query to use for the given arguments.
# File lib/sequel/dataset/placeholder_literalizer.rb, line 151 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
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