Module: Sequel::Model::Associations::DatasetMethods

Defined in:
lib/sequel/model/associations.rb

Overview

Eager loading makes it so that you can load all associated records for a set of objects in a single query, instead of a separate query for each object.

Two separate implementations are provided. eager should be used most of the time, as it loads associated records using one query per association. However, it does not allow you the ability to filter or order based on columns in associated tables. eager_graph loads all records in a single query using JOINs, allowing you to filter or order based on columns in associated tables. However, eager_graph can be slower than eager, especially if multiple one_to_many or many_to_many associations are joined.

You can cascade the eager loading (loading associations on associated objects) with no limit to the depth of the cascades. You do this by passing a hash to eager or eager_graph with the keys being associations of the current model and values being associations of the model associated with the current model via the key.

The arguments can be symbols or hashes with symbol keys (for cascaded eager loading). Examples:

Album.eager(:artist).all
Album.eager_graph(:artist).all
Album.eager(:artist, :genre).all
Album.eager_graph(:artist, :genre).all
Album.eager(:artist).eager(:genre).all
Album.eager_graph(:artist).eager(:genre).all
Artist.eager(:albums=>:tracks).all
Artist.eager_graph(:albums=>:tracks).all
Artist.eager(:albums=>{:tracks=>:genre}).all
Artist.eager_graph(:albums=>{:tracks=>:genre}).all

You can also pass a callback as a hash value in order to customize the dataset being eager loaded at query time, analogous to the way the :eager_block association option allows you to customize it at association definition time. For example, if you wanted artists with their albums since 1990:

Artist.eager(:albums => proc{|ds| ds.filter{year > 1990}})

Or if you needed albums and their artist’s name only, using a single query:

Albums.eager_graph(:artist => proc{|ds| ds.select(:name)})

To cascade eager loading while using a callback, you substitute the cascaded associations with a single entry hash that has the proc callback as the key and the cascaded associations as the value. This will load artists with their albums since 1990, and also the tracks on those albums and the genre for those tracks:

Artist.eager(:albums => {proc{|ds| ds.filter{year > 1990}}=>{:tracks => :genre}})

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.extended(obj) ⇒ Object

Add the eager! and eager_graph! mutation methods to the dataset.



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# File 'lib/sequel/model/associations.rb', line 1329

def self.extended(obj)
  obj.def_mutation_method(:eager, :eager_graph)
end

Instance Method Details

#complex_expression_sql(op, args) ⇒ Object

If the expression is in the form x = y where y is a Sequel::Model instance, assume x is an association symbol and look up the association reflection via the dataset’s model. From there, return the appropriate SQL based on the type of association and the values of the foreign/primary keys of y. For most association types, this is a simple transformation, but for many_to_many associations this creates a subquery to the join table.



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# File 'lib/sequel/model/associations.rb', line 1417

def complex_expression_sql(op, args)
  if op == :'=' and args.at(1).is_a?(Sequel::Model)
    l, r = args
    if ar = model.association_reflections[l]
      unless r.is_a?(ar.associated_class)
        raise Sequel::Error, "invalid association class #{r.class.inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}, expected class #{ar.associated_class.inspect}"
      end

      if exp = association_filter_expression(ar, r)
        literal(exp)
      else
        raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}"
      end
    else
      raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}"
    end
  else
    super
  end
end

#eager(*associations) ⇒ Object

The preferred eager loading method. Loads all associated records using one query for each association.

The basic idea for how it works is that the dataset is first loaded normally. Then it goes through all associations that have been specified via eager. It loads each of those associations separately, then associates them back to the original dataset via primary/foreign keys. Due to the necessity of all objects being present, you need to use all to use eager loading, as it can’t work with each.

This implementation avoids the complexity of extracting an object graph out of a single dataset, by building the object graph out of multiple datasets, one for each association. By using a separate dataset for each association, it avoids problems such as aliasing conflicts and creating cartesian product result sets if multiple one_to_many or many_to_many eager associations are requested.

One limitation of using this method is that you cannot filter the dataset based on values of columns in an associated table, since the associations are loaded in separate queries. To do that you need to load all associations in the same query, and extract an object graph from the results of that query. If you need to filter based on columns in associated tables, look at eager_graph or join the tables you need to filter on manually.

Each association’s order, if defined, is respected. Eager also works on a limited dataset, but does not use any :limit options for associations. If the association uses a block or has an :eager_block argument, it is used.



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# File 'lib/sequel/model/associations.rb', line 1359

def eager(*associations)
  opt = @opts[:eager]
  opt = opt ? opt.dup : {}
  associations.flatten.each do |association|
    case association
      when Symbol
        check_association(model, association)
        opt[association] = nil
      when Hash
        association.keys.each{|assoc| check_association(model, assoc)}
        opt.merge!(association)
      else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash')
    end
  end
  clone(:eager=>opt)
end

#eager_graph(*associations) ⇒ Object

The secondary eager loading method. Loads all associations in a single query. This method should only be used if you need to filter or order based on columns in associated tables.

This method builds an object graph using Dataset#graph. Then it uses the graph to build the associations, and finally replaces the graph with a simple array of model objects.

Be very careful when using this with multiple one_to_many or many_to_many associations, as you can create large cartesian products. If you must graph multiple one_to_many and many_to_many associations, make sure your filters are narrow if you have a large database.

Each association’s order, if definied, is respected. eager_graph probably won’t work correctly on a limited dataset, unless you are only graphing many_to_one and one_to_one associations.

Does not use the block defined for the association, since it does a single query for all objects. You can use the :graph_* association options to modify the SQL query.

Like eager, you need to call all on the dataset for the eager loading to work. If you just call each, you will get a normal graphed result back (a hash with table alias symbol keys and model object values).



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# File 'lib/sequel/model/associations.rb', line 1397

def eager_graph(*associations)
  ds = if @opts[:eager_graph]
    self
  else
    # Each of the following have a symbol key for the table alias, with the following values: 
    # :reciprocals - the reciprocal instance variable to use for this association
    # :requirements - array of requirements for this association
    # :alias_association_type_map - the type of association for this association
    # :alias_association_name_map - the name of the association for this association
    clone(:eager_graph=>{:requirements=>{}, :master=>alias_symbol(first_source), :alias_association_type_map=>{}, :alias_association_name_map=>{}, :reciprocals=>{}, :cartesian_product_number=>0})
  end
  ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations)
end

#ungraphed ⇒ Object

Do not attempt to split the result set into associations, just return results as simple objects. This is useful if you want to use eager_graph as a shortcut to have all of the joins and aliasing set up, but want to do something else with the dataset.



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# File 'lib/sequel/model/associations.rb', line 1442

def ungraphed
  super.clone(:eager_graph=>nil)
end