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 is usually 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}})
Add the eager! and eager_graph! mutation methods to the dataset.
# File lib/sequel/model/associations.rb, line 1728 1728: def self.extended(obj) 1729: obj.def_mutation_method(:eager, :eager_graph) 1730: end
If the expression is in the form x = y where y is a Sequel::Model instance, array of Sequel::Model instances, or a Sequel::Model dataset, 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.
# File lib/sequel/model/associations.rb, line 1739 1739: def complex_expression_sql_append(sql, op, args) 1740: r = args.at(1) 1741: if (((op == :'=' || op == :'!=') and r.is_a?(Sequel::Model)) || 1742: (multiple = ((op == :IN || op == :'NOT IN') and ((is_ds = r.is_a?(Sequel::Dataset)) or r.all?{|x| x.is_a?(Sequel::Model)})))) 1743: l = args.at(0) 1744: if ar = model.association_reflections[l] 1745: if multiple 1746: klass = ar.associated_class 1747: if is_ds 1748: if r.respond_to?(:model) 1749: unless r.model <= klass 1750: # A dataset for a different model class, could be a valid regular query 1751: return super 1752: end 1753: else 1754: # Not a model dataset, could be a valid regular query 1755: return super 1756: end 1757: else 1758: unless r.all?{|x| x.is_a?(klass)} 1759: raise Sequel::Error, "invalid association class for one object for association #{l.inspect} used in dataset filter for model #{model.inspect}, expected class #{klass.inspect}" 1760: end 1761: end 1762: elsif !r.is_a?(ar.associated_class) 1763: 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}" 1764: end 1765: 1766: if exp = association_filter_expression(op, ar, r) 1767: literal_append(sql, exp) 1768: else 1769: raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}" 1770: end 1771: elsif multiple && (is_ds || r.empty?) 1772: # Not a query designed for this support, could be a valid regular query 1773: super 1774: else 1775: raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}" 1776: end 1777: else 1778: super 1779: end 1780: end
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.
# File lib/sequel/model/associations.rb, line 1808 1808: def eager(*associations) 1809: opt = @opts[:eager] 1810: opt = opt ? opt.dup : {} 1811: associations.flatten.each do |association| 1812: case association 1813: when Symbol 1814: check_association(model, association) 1815: opt[association] = nil 1816: when Hash 1817: association.keys.each{|assoc| check_association(model, assoc)} 1818: opt.merge!(association) 1819: else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 1820: end 1821: end 1822: clone(:eager=>opt) 1823: end
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 uses Dataset#graph to create appropriate aliases for columns in all the tables. Then it uses the graph’s metadata to build the associations from the single hash, and finally replaces the array of hashes with an array model objects inside all.
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, it will yield plain hashes, each containing all columns from all the tables.
# File lib/sequel/model/associations.rb, line 1845 1845: def eager_graph(*associations) 1846: ds = if eg = @opts[:eager_graph] 1847: eg = eg.dup 1848: [:requirements, :reflections, :reciprocals].each{|k| eg[k] = eg[k].dup} 1849: clone(:eager_graph=>eg) 1850: else 1851: # Each of the following have a symbol key for the table alias, with the following values: 1852: # :reciprocals - the reciprocal instance variable to use for this association 1853: # :reflections - AssociationReflection instance related to this association 1854: # :requirements - array of requirements for this association 1855: clone(:eager_graph=>{:requirements=>{}, :master=>alias_symbol(first_source), :reflections=>{}, :reciprocals=>{}, :cartesian_product_number=>0}) 1856: end 1857: ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations) 1858: end
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.
# File lib/sequel/model/associations.rb, line 1864 1864: def ungraphed 1865: super.clone(:eager_graph=>nil) 1866: end
Call graph on the association with the correct arguments, update the eager_graph data structure, and recurse into eager_graph_associations if there are any passed in associations (which would be dependencies of the current association)
Arguments:
ds | Current dataset |
model | Current Model |
ta | table_alias used for the parent association |
requirements | an array, used as a stack for requirements |
r | association reflection for the current association |
*associations | any associations dependent on this one |
# File lib/sequel/model/associations.rb, line 1882 1882: def eager_graph_association(ds, model, ta, requirements, r, *associations) 1883: assoc_name = r[:name] 1884: assoc_table_alias = ds.unused_table_alias(r[:graph_alias_base]) 1885: loader = r[:eager_grapher] 1886: if !associations.empty? 1887: if associations.first.respond_to?(:call) 1888: callback = associations.first 1889: associations = {} 1890: elsif associations.length == 1 && (assocs = associations.first).is_a?(Hash) && assocs.length == 1 && (pr_assoc = assocs.to_a.first) && pr_assoc.first.respond_to?(:call) 1891: callback, assoc = pr_assoc 1892: associations = assoc.is_a?(Array) ? assoc : [assoc] 1893: end 1894: end 1895: ds = if loader.arity == 1 1896: loader.call(:self=>ds, :table_alias=>assoc_table_alias, :implicit_qualifier=>ta, :callback=>callback) 1897: else 1898: loader.call(ds, assoc_table_alias, ta) 1899: end 1900: ds = ds.order_more(*qualified_expression(r[:order], assoc_table_alias)) if r[:order] and r[:order_eager_graph] 1901: eager_graph = ds.opts[:eager_graph] 1902: eager_graph[:requirements][assoc_table_alias] = requirements.dup 1903: eager_graph[:reflections][assoc_table_alias] = r 1904: eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2 1905: ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty? 1906: ds 1907: end
Check the associations are valid for the given model. Call eager_graph_association on each association.
Arguments:
ds | Current dataset |
model | Current Model |
ta | table_alias used for the parent association |
requirements | an array, used as a stack for requirements |
*associations | the associations to add to the graph |
# File lib/sequel/model/associations.rb, line 1918 1918: def eager_graph_associations(ds, model, ta, requirements, *associations) 1919: return ds if associations.empty? 1920: associations.flatten.each do |association| 1921: ds = case association 1922: when Symbol 1923: ds.eager_graph_association(ds, model, ta, requirements, check_association(model, association)) 1924: when Hash 1925: association.each do |assoc, assoc_assocs| 1926: ds = ds.eager_graph_association(ds, model, ta, requirements, check_association(model, assoc), assoc_assocs) 1927: end 1928: ds 1929: else raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 1930: end 1931: end 1932: ds 1933: end
Replace the array of plain hashes with an array of model objects will all eager_graphed associations set in the associations cache for each object.
# File lib/sequel/model/associations.rb, line 1937 1937: def eager_graph_build_associations(hashes) 1938: hashes.replace(EagerGraphLoader.new(self).load(hashes)) 1939: end
Return an expression for filtering by the given association reflection and associated object.
# File lib/sequel/model/associations.rb, line 1944 1944: def association_filter_expression(op, ref, obj) 1945: meth = :"#{ref[:type]}_association_filter_expression" 1946: send(meth, op, ref, obj) if respond_to?(meth, true) 1947: end
Handle inversion for association filters by returning an inverted expression, plus also handling cases where the referenced columns are NULL.
# File lib/sequel/model/associations.rb, line 1951 1951: def association_filter_handle_inversion(op, exp, cols) 1952: if op == :'!=' || op == :'NOT IN' 1953: if exp == SQL::Constants::FALSE 1954: ~exp 1955: else 1956: ~exp | Sequel::SQL::BooleanExpression.from_value_pairs(cols.zip([]), :OR) 1957: end 1958: else 1959: exp 1960: end 1961: end
Return an expression for making sure that the given keys match the value of the given methods for either the single object given or for any of the objects given if obj is an array.
# File lib/sequel/model/associations.rb, line 1966 1966: def association_filter_key_expression(keys, meths, obj) 1967: vals = if obj.is_a?(Sequel::Dataset) 1968: {(keys.length == 1 ? keys.first : keys)=>obj.select(*meths).exclude(Sequel::SQL::BooleanExpression.from_value_pairs(meths.zip([]), :OR))} 1969: else 1970: vals = Array(obj).reject{|o| !meths.all?{|m| o.send(m)}} 1971: return SQL::Constants::FALSE if vals.empty? 1972: if obj.is_a?(Array) 1973: if keys.length == 1 1974: meth = meths.first 1975: {keys.first=>vals.map{|o| o.send(meth)}} 1976: else 1977: {keys=>vals.map{|o| meths.map{|m| o.send(m)}}} 1978: end 1979: else 1980: keys.zip(meths.map{|k| obj.send(k)}) 1981: end 1982: end 1983: SQL::BooleanExpression.from_value_pairs(vals) 1984: end
Make sure the association is valid for this model, and return the related AssociationReflection.
# File lib/sequel/model/associations.rb, line 1987 1987: def check_association(model, association) 1988: raise(Sequel::UndefinedAssociation, "Invalid association #{association} for #{model.name}") unless reflection = model.association_reflection(association) 1989: raise(Sequel::Error, "Eager loading is not allowed for #{model.name} association #{association}") if reflection[:allow_eager] == false 1990: reflection 1991: end
Eagerly load all specified associations
# File lib/sequel/model/associations.rb, line 1994 1994: def eager_load(a, eager_assoc=@opts[:eager]) 1995: return if a.empty? 1996: # Key is foreign/primary key name symbol 1997: # Value is hash with keys being foreign/primary key values (generally integers) 1998: # and values being an array of current model objects with that 1999: # specific foreign/primary key 2000: key_hash = {} 2001: # Reflections for all associations to eager load 2002: reflections = eager_assoc.keys.collect{|assoc| model.association_reflection(assoc) || (raise Sequel::UndefinedAssociation, "Model: #{self}, Association: #{assoc}")} 2003: 2004: # Populate keys to monitor 2005: reflections.each{|reflection| key_hash[reflection.eager_loader_key] ||= Hash.new{|h,k| h[k] = []}} 2006: 2007: # Associate each object with every key being monitored 2008: a.each do |rec| 2009: key_hash.each do |key, id_map| 2010: case key 2011: when Array 2012: id_map[key.map{|k| rec[k]}] << rec if key.all?{|k| rec[k]} 2013: when Symbol 2014: id_map[rec[key]] << rec if rec[key] 2015: end 2016: end 2017: end 2018: 2019: reflections.each do |r| 2020: loader = r[:eager_loader] 2021: associations = eager_assoc[r[:name]] 2022: if associations.respond_to?(:call) 2023: eager_block = associations 2024: associations = {} 2025: elsif associations.is_a?(Hash) && associations.length == 1 && (pr_assoc = associations.to_a.first) && pr_assoc.first.respond_to?(:call) 2026: eager_block, associations = pr_assoc 2027: end 2028: if loader.arity == 1 2029: loader.call(:key_hash=>key_hash, :rows=>a, :associations=>associations, :self=>self, :eager_block=>eager_block) 2030: else 2031: loader.call(key_hash, a, associations) 2032: end 2033: a.each{|object| object.send(:run_association_callbacks, r, :after_load, object.associations[r[:name]])} unless r[:after_load].empty? 2034: end 2035: end
Return plain hashes instead of calling the row_proc if eager_graph is being used.
# File lib/sequel/model/associations.rb, line 2038 2038: def graph_each(&block) 2039: @opts[:eager_graph] ? fetch_rows(select_sql, &block) : super 2040: end
Return a subquery expression for filering by a many_to_many association
# File lib/sequel/model/associations.rb, line 2043 2043: def many_to_many_association_filter_expression(op, ref, obj) 2044: lpks, lks, rks = ref.values_at(:left_primary_keys, :left_keys, :right_keys) 2045: jt = ref.join_table_alias 2046: lpks = lpks.first if lpks.length == 1 2047: lpks = ref.qualify(model.table_name, lpks) 2048: meths = ref.right_primary_keys 2049: meths = ref.qualify(obj.model.table_name, meths) if obj.is_a?(Sequel::Dataset) 2050: exp = association_filter_key_expression(ref.qualify(jt, rks), meths, obj) 2051: if exp == SQL::Constants::FALSE 2052: association_filter_handle_inversion(op, exp, Array(lpks)) 2053: else 2054: association_filter_handle_inversion(op, SQL::BooleanExpression.from_value_pairs(lpks=>model.db.from(ref[:join_table]).select(*ref.qualify(jt, lks)).where(exp).exclude(SQL::BooleanExpression.from_value_pairs(ref.qualify(jt, lks).zip([]), :OR))), Array(lpks)) 2055: end 2056: end
Return a simple equality expression for filering by a many_to_one association
# File lib/sequel/model/associations.rb, line 2059 2059: def many_to_one_association_filter_expression(op, ref, obj) 2060: keys = ref.qualify(model.table_name, ref[:keys]) 2061: meths = ref.primary_keys 2062: meths = ref.qualify(obj.model.table_name, meths) if obj.is_a?(Sequel::Dataset) 2063: association_filter_handle_inversion(op, association_filter_key_expression(keys, meths, obj), keys) 2064: end
Return a simple equality expression for filering by a one_to_* association
# File lib/sequel/model/associations.rb, line 2067 2067: def one_to_many_association_filter_expression(op, ref, obj) 2068: keys = ref.qualify(model.table_name, ref[:primary_keys]) 2069: meths = ref[:keys] 2070: meths = ref.qualify(obj.model.table_name, meths) if obj.is_a?(Sequel::Dataset) 2071: association_filter_handle_inversion(op, association_filter_key_expression(keys, meths, obj), keys) 2072: end
Build associations from the graph if # was used, and/or load other associations if # was used.
# File lib/sequel/model/associations.rb, line 2077 2077: def post_load(all_records) 2078: eager_graph_build_associations(all_records) if @opts[:eager_graph] 2079: eager_load(all_records) if @opts[:eager] 2080: super 2081: end
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