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_graph(: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.where{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.where{year > 1990}}=>{tracks: :genre}})
Methods
Public Instance
- as_hash
- association_join
- complex_expression_sql_append
- eager
- eager_graph
- eager_graph_with_options
- to_hash_groups
- ungraphed
Protected Instance
Public Instance methods
If the dataset is being eagerly loaded, default to calling all instead of each.
# File lib/sequel/model/associations.rb 3139 def as_hash(key_column=nil, value_column=nil, opts=OPTS) 3140 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3141 opts = Hash[opts] 3142 opts[:all] = true 3143 end 3144 super 3145 end
Adds one or more INNER JOINs to the existing dataset using the keys and conditions specified by the given association(s). Take the same arguments as eager_graph, and operates similarly, but only adds the joins as opposed to making the other changes (such as adding selected columns and setting up eager loading).
The following methods also exist for specifying a different type of JOIN:
| association_full_join |
FULL JOIN |
| association_inner_join |
INNER JOIN |
| association_left_join |
LEFT JOIN |
| association_right_join |
RIGHT JOIN |
Examples:
# For each album, association_join load the artist Album.association_join(:artist).all # SELECT * # FROM albums # INNER JOIN artists AS artist ON (artists.id = albums.artist_id) # For each album, association_join load the artist, using a specified alias Album.association_join(Sequel[:artist].as(:a)).all # SELECT * # FROM albums # INNER JOIN artists AS a ON (a.id = albums.artist_id) # For each album, association_join load the artist and genre Album.association_join(:artist, :genre).all Album.association_join(:artist).association_join(:genre).all # SELECT * # FROM albums # INNER JOIN artists AS artist ON (artist.id = albums.artist_id) # INNER JOIN genres AS genre ON (genre.id = albums.genre_id) # For each artist, association_join load albums and tracks for each album Artist.association_join(albums: :tracks).all # SELECT * # FROM artists # INNER JOIN albums ON (albums.artist_id = artists.id) # INNER JOIN tracks ON (tracks.album_id = albums.id) # For each artist, association_join load albums, tracks for each album, and genre for each track Artist.association_join(albums: {tracks: :genre}).all # SELECT * # FROM artists # INNER JOIN albums ON (albums.artist_id = artists.id) # INNER JOIN tracks ON (tracks.album_id = albums.id) # INNER JOIN genres AS genre ON (genre.id = tracks.genre_id) # For each artist, association_join load albums with year > 1990 Artist.association_join(albums: proc{|ds| ds.where{year > 1990}}).all # SELECT * # FROM artists # INNER JOIN ( # SELECT * FROM albums WHERE (year > 1990) # ) AS albums ON (albums.artist_id = artists.id) # For each artist, association_join load albums and tracks 1-10 for each album Artist.association_join(albums: {tracks: proc{|ds| ds.where(number: 1..10)}}).all # SELECT * # FROM artists # INNER JOIN albums ON (albums.artist_id = artists.id) # INNER JOIN ( # SELECT * FROM tracks WHERE ((number >= 1) AND (number <= 10)) # ) AS tracks ON (tracks.albums_id = albums.id) # For each artist, association_join load albums with year > 1990, and tracks for those albums Artist.association_join(albums: {proc{|ds| ds.where{year > 1990}}=>:tracks}).all # SELECT * # FROM artists # INNER JOIN ( # SELECT * FROM albums WHERE (year > 1990) # ) AS albums ON (albums.artist_id = artists.id) # INNER JOIN tracks ON (tracks.album_id = albums.id)
# File lib/sequel/model/associations.rb 2847 def association_join(*associations) 2848 association_inner_join(*associations) 2849 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 2858 def complex_expression_sql_append(sql, op, args) 2859 r = args[1] 2860 if (((op == :'=' || op == :'!=') && r.is_a?(Sequel::Model)) || 2861 (multiple = ((op == :IN || op == :'NOT IN') && ((is_ds = r.is_a?(Sequel::Dataset)) || (r.respond_to?(:all?) && r.all?{|x| x.is_a?(Sequel::Model)}))))) 2862 l = args[0] 2863 if ar = model.association_reflections[l] 2864 if multiple 2865 klass = ar.associated_class 2866 if is_ds 2867 if r.respond_to?(:model) 2868 unless r.model <= klass 2869 # A dataset for a different model class, could be a valid regular query 2870 return super 2871 end 2872 else 2873 # Not a model dataset, could be a valid regular query 2874 return super 2875 end 2876 else 2877 unless r.all?{|x| x.is_a?(klass)} 2878 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}" 2879 end 2880 end 2881 elsif !r.is_a?(ar.associated_class) 2882 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}" 2883 end 2884 2885 if exp = association_filter_expression(op, ar, r) 2886 literal_append(sql, exp) 2887 else 2888 raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}" 2889 end 2890 elsif multiple && (is_ds || r.empty?) 2891 # Not a query designed for this support, could be a valid regular query 2892 super 2893 else 2894 raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}" 2895 end 2896 else 2897 super 2898 end 2899 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 current 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. If the association uses a block or has an :eager_block argument, it is used.
To modify the associated dataset that will be used for the eager load, you should use a hash for the association, with the key being the association name symbol, and the value being a callable object that is called with the associated dataset and should return a modified dataset. If that association also has dependent associations, instead of a callable object, use a hash with the callable object being the key, and the dependent association(s) as the value.
Examples:
# For each album, eager load the artist Album.eager(:artist).all # SELECT * FROM albums # SELECT * FROM artists WHERE (id IN (...)) # For each album, eager load the artist and genre Album.eager(:artist, :genre).all Album.eager(:artist).eager(:genre).all # SELECT * FROM albums # SELECT * FROM artists WHERE (id IN (...)) # SELECT * FROM genres WHERE (id IN (...)) # For each artist, eager load albums and tracks for each album Artist.eager(albums: :tracks).all # SELECT * FROM artists # SELECT * FROM albums WHERE (artist_id IN (...)) # SELECT * FROM tracks WHERE (album_id IN (...)) # For each artist, eager load albums, tracks for each album, and genre for each track Artist.eager(albums: {tracks: :genre}).all # SELECT * FROM artists # SELECT * FROM albums WHERE (artist_id IN (...)) # SELECT * FROM tracks WHERE (album_id IN (...)) # SELECT * FROM genre WHERE (id IN (...)) # For each artist, eager load albums with year > 1990 Artist.eager(albums: proc{|ds| ds.where{year > 1990}}).all # SELECT * FROM artists # SELECT * FROM albums WHERE ((year > 1990) AND (artist_id IN (...))) # For each artist, eager load albums and tracks 1-10 for each album Artist.eager(albums: {tracks: proc{|ds| ds.where(number: 1..10)}}).all # SELECT * FROM artists # SELECT * FROM albums WHERE (artist_id IN (...)) # SELECT * FROM tracks WHERE ((number >= 1) AND (number <= 10) AND (album_id IN (...))) # For each artist, eager load albums with year > 1990, and tracks for those albums Artist.eager(albums: {proc{|ds| ds.where{year > 1990}}=>:tracks}).all # SELECT * FROM artists # SELECT * FROM albums WHERE ((year > 1990) AND (artist_id IN (...))) # SELECT * FROM albums WHERE (artist_id IN (...))
# File lib/sequel/model/associations.rb 2976 def eager(*associations) 2977 opts = @opts[:eager] 2978 association_opts = eager_options_for_associations(associations) 2979 opts = opts ? opts.merge(association_opts) : association_opts 2980 clone(:eager=>opts.freeze) 2981 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, or if you have done comparative benchmarking it and determined it is faster.
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 the datasets are large.
Each association's order, if defined, is respected. eager_graph probably won't work correctly on a limited dataset, unless you are only graphing many_to_one, one_to_one, and one_through_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.
To modify the associated dataset that will be joined to the current dataset, you should use a hash for the association, with the key being the association name symbol, and the value being a callable object that is called with the associated dataset and should return a modified dataset. If that association also has dependent associations, instead of a callable object, use a hash with the callable object being the key, and the dependent association(s) as the value.
You can specify an custom alias and/or join type on a per-association basis by providing an Sequel::SQL::AliasedExpression object instead of an a Symbol for the association name.
You cannot mix calls to eager_graph and graph on the same dataset.
Examples:
# For each album, eager_graph load the artist Album.eager_graph(:artist).all # SELECT ... # FROM albums # LEFT OUTER JOIN artists AS artist ON (artists.id = albums.artist_id) # For each album, eager_graph load the artist, using a specified alias Album.eager_graph(Sequel[:artist].as(:a)).all # SELECT ... # FROM albums # LEFT OUTER JOIN artists AS a ON (a.id = albums.artist_id) # For each album, eager_graph load the artist, using a specified alias # and custom join type Album.eager_graph(Sequel[:artist].as(:a, join_type: :inner)).all # SELECT ... # FROM albums # INNER JOIN artists AS a ON (a.id = albums.artist_id) # For each album, eager_graph load the artist and genre Album.eager_graph(:artist, :genre).all Album.eager_graph(:artist).eager_graph(:genre).all # SELECT ... # FROM albums # LEFT OUTER JOIN artists AS artist ON (artist.id = albums.artist_id) # LEFT OUTER JOIN genres AS genre ON (genre.id = albums.genre_id) # For each artist, eager_graph load albums and tracks for each album Artist.eager_graph(albums: :tracks).all # SELECT ... # FROM artists # LEFT OUTER JOIN albums ON (albums.artist_id = artists.id) # LEFT OUTER JOIN tracks ON (tracks.album_id = albums.id) # For each artist, eager_graph load albums, tracks for each album, and genre for each track Artist.eager_graph(albums: {tracks: :genre}).all # SELECT ... # FROM artists # LEFT OUTER JOIN albums ON (albums.artist_id = artists.id) # LEFT OUTER JOIN tracks ON (tracks.album_id = albums.id) # LEFT OUTER JOIN genres AS genre ON (genre.id = tracks.genre_id) # For each artist, eager_graph load albums with year > 1990 Artist.eager_graph(albums: proc{|ds| ds.where{year > 1990}}).all # SELECT ... # FROM artists # LEFT OUTER JOIN ( # SELECT * FROM albums WHERE (year > 1990) # ) AS albums ON (albums.artist_id = artists.id) # For each artist, eager_graph load albums and tracks 1-10 for each album Artist.eager_graph(albums: {tracks: proc{|ds| ds.where(number: 1..10)}}).all # SELECT ... # FROM artists # LEFT OUTER JOIN albums ON (albums.artist_id = artists.id) # LEFT OUTER JOIN ( # SELECT * FROM tracks WHERE ((number >= 1) AND (number <= 10)) # ) AS tracks ON (tracks.albums_id = albums.id) # For each artist, eager_graph load albums with year > 1990, and tracks for those albums Artist.eager_graph(albums: {proc{|ds| ds.where{year > 1990}}=>:tracks}).all # SELECT ... # FROM artists # LEFT OUTER JOIN ( # SELECT * FROM albums WHERE (year > 1990) # ) AS albums ON (albums.artist_id = artists.id) # LEFT OUTER JOIN tracks ON (tracks.album_id = albums.id)
# File lib/sequel/model/associations.rb 3086 def eager_graph(*associations) 3087 eager_graph_with_options(associations) 3088 end
Run eager_graph with some options specific to just this call. Unlike eager_graph, this takes the associations as a single argument instead of multiple arguments.
Options:
| :join_type |
Override the join type specified in the association | ||||||||||
| :limit_strategy |
Use a strategy for handling limits on associations. Appropriate :limit_strategy values are:
This can also be a hash with association name symbol keys and one of the above values, to use different strategies per association. The default is the :ruby strategy. Choosing a different strategy can make your code significantly slower in some cases (perhaps even the majority of cases), so you should only use this if you have benchmarked that it is faster for your use cases. |
# File lib/sequel/model/associations.rb 3110 def eager_graph_with_options(associations, opts=OPTS) 3111 return self if associations.empty? 3112 3113 opts = opts.dup unless opts.frozen? 3114 associations = [associations] unless associations.is_a?(Array) 3115 ds = if eg = @opts[:eager_graph] 3116 eg = eg.dup 3117 [:requirements, :reflections, :reciprocals, :limits].each{|k| eg[k] = eg[k].dup} 3118 eg[:local] = opts 3119 ds = clone(:eager_graph=>eg) 3120 ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations) 3121 else 3122 # Each of the following have a symbol key for the table alias, with the following values: 3123 # :reciprocals :: the reciprocal value to use for this association 3124 # :reflections :: AssociationReflection instance related to this association 3125 # :requirements :: array of requirements for this association 3126 # :limits :: Any limit/offset array slicing that need to be handled in ruby land after loading 3127 opts = {:requirements=>{}, :master=>alias_symbol(first_source), :reflections=>{}, :reciprocals=>{}, :limits=>{}, :local=>opts, :cartesian_product_number=>0, :row_proc=>row_proc} 3128 ds = clone(:eager_graph=>opts) 3129 ds = ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations).naked 3130 end 3131 3132 ds.opts[:eager_graph].freeze 3133 ds.opts[:eager_graph].each_value{|v| v.freeze if v.is_a?(Hash)} 3134 ds 3135 end
If the dataset is being eagerly loaded, default to calling all instead of each.
# File lib/sequel/model/associations.rb 3149 def to_hash_groups(key_column, value_column=nil, opts=OPTS) 3150 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3151 opts = Hash[opts] 3152 opts[:all] = true 3153 end 3154 super 3155 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 3161 def ungraphed 3162 ds = super.clone(:eager_graph=>nil) 3163 if (eg = @opts[:eager_graph]) && (rp = eg[:row_proc]) 3164 ds = ds.with_row_proc(rp) 3165 end 3166 ds 3167 end
Protected Instance methods
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 |
| ta |
table_alias used for the parent association |
| requirements |
an array, used as a stack for requirements |
| r |
association reflection for the current association, or an |
| *associations |
any associations dependent on this one |
# File lib/sequel/model/associations.rb 3186 def eager_graph_association(ds, model, ta, requirements, r, *associations) 3187 if r.is_a?(SQL::AliasedExpression) 3188 alias_base = r.alias 3189 if r.columns.is_a?(Hash) 3190 join_type = r.columns[:join_type] 3191 end 3192 r = r.expression 3193 else 3194 alias_base = r[:graph_alias_base] 3195 end 3196 assoc_table_alias = ds.unused_table_alias(alias_base) 3197 loader = r[:eager_grapher] 3198 if !associations.empty? 3199 if associations.first.respond_to?(:call) 3200 callback = associations.first 3201 associations = {} 3202 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) 3203 callback, assoc = pr_assoc 3204 associations = assoc.is_a?(Array) ? assoc : [assoc] 3205 end 3206 end 3207 local_opts = ds.opts[:eager_graph][:local] 3208 limit_strategy = r.eager_graph_limit_strategy(local_opts[:limit_strategy]) 3209 3210 if r[:conditions] && !Sequel.condition_specifier?(r[:conditions]) && !r[:orig_opts].has_key?(:graph_conditions) && !r[:orig_opts].has_key?(:graph_only_conditions) && !r.has_key?(:graph_block) 3211 raise Error, "Cannot eager_graph association when :conditions specified and not a hash or an array of pairs. Specify :graph_conditions, :graph_only_conditions, or :graph_block for the association. Model: #{r[:model]}, association: #{r[:name]}" 3212 end 3213 3214 ds = loader.call(:self=>ds, :table_alias=>assoc_table_alias, :implicit_qualifier=>(ta == ds.opts[:eager_graph][:master]) ? first_source : qualifier_from_alias_symbol(ta, first_source), :callback=>callback, :join_type=>join_type || local_opts[:join_type], :join_only=>local_opts[:join_only], :limit_strategy=>limit_strategy, :from_self_alias=>ds.opts[:eager_graph][:master]) 3215 if r[:order_eager_graph] && (order = r.fetch(:graph_order, r[:order])) 3216 ds = ds.order_append(*qualified_expression(order, assoc_table_alias)) 3217 end 3218 eager_graph = ds.opts[:eager_graph] 3219 eager_graph[:requirements][assoc_table_alias] = requirements.dup 3220 eager_graph[:reflections][assoc_table_alias] = r 3221 if limit_strategy == :ruby 3222 eager_graph[:limits][assoc_table_alias] = r.limit_and_offset 3223 end 3224 eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2 3225 ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty? 3226 ds 3227 end
Check the associations are valid for the given model. Call eager_graph_association on each association.
Arguments:
| ds |
Current dataset |
| model |
Current |
| 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 3238 def eager_graph_associations(ds, model, ta, requirements, *associations) 3239 associations.flatten.each do |association| 3240 ds = case association 3241 when Symbol, SQL::AliasedExpression 3242 ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, association)) 3243 when Hash 3244 association.each do |assoc, assoc_assocs| 3245 ds = ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, assoc), assoc_assocs) 3246 end 3247 ds 3248 else 3249 raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 3250 end 3251 end 3252 ds 3253 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 3257 def eager_graph_build_associations(hashes) 3258 hashes.replace(_eager_graph_build_associations(hashes, eager_graph_loader)) 3259 end