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 3287 def as_hash(key_column=nil, value_column=nil, opts=OPTS) 3288 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3289 opts = Hash[opts] 3290 opts[:all] = true 3291 end 3292 super 3293 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 2993 def association_join(*associations) 2994 association_inner_join(*associations) 2995 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 3004 def complex_expression_sql_append(sql, op, args) 3005 r = args[1] 3006 if (((op == :'=' || op == :'!=') && r.is_a?(Sequel::Model)) || 3007 (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)}))))) 3008 l = args[0] 3009 if ar = model.association_reflections[l] 3010 raise Error, "filtering by associations is not allowed for #{ar.inspect}" if ar[:allow_filtering_by] == false 3011 3012 if multiple 3013 klass = ar.associated_class 3014 if is_ds 3015 if r.respond_to?(:model) 3016 unless r.model <= klass 3017 # A dataset for a different model class, could be a valid regular query 3018 return super 3019 end 3020 else 3021 # Not a model dataset, could be a valid regular query 3022 return super 3023 end 3024 else 3025 unless r.all?{|x| x.is_a?(klass)} 3026 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}" 3027 end 3028 end 3029 elsif !r.is_a?(ar.associated_class) 3030 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}" 3031 end 3032 3033 if exp = association_filter_expression(op, ar, r) 3034 literal_append(sql, exp) 3035 else 3036 raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}" 3037 end 3038 elsif multiple && (is_ds || r.empty?) 3039 # Not a query designed for this support, could be a valid regular query 3040 super 3041 else 3042 raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}" 3043 end 3044 else 3045 super 3046 end 3047 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 3124 def eager(*associations) 3125 opts = @opts[:eager] 3126 association_opts = eager_options_for_associations(associations) 3127 opts = opts ? opts.merge(association_opts) : association_opts 3128 clone(:eager=>opts.freeze) 3129 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 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 3234 def eager_graph(*associations) 3235 eager_graph_with_options(associations) 3236 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 3258 def eager_graph_with_options(associations, opts=OPTS) 3259 return self if associations.empty? 3260 3261 opts = opts.dup unless opts.frozen? 3262 associations = [associations] unless associations.is_a?(Array) 3263 ds = if eg = @opts[:eager_graph] 3264 eg = eg.dup 3265 [:requirements, :reflections, :reciprocals, :limits].each{|k| eg[k] = eg[k].dup} 3266 eg[:local] = opts 3267 ds = clone(:eager_graph=>eg) 3268 ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations) 3269 else 3270 # Each of the following have a symbol key for the table alias, with the following values: 3271 # :reciprocals :: the reciprocal value to use for this association 3272 # :reflections :: AssociationReflection instance related to this association 3273 # :requirements :: array of requirements for this association 3274 # :limits :: Any limit/offset array slicing that need to be handled in ruby land after loading 3275 opts = {:requirements=>{}, :master=>alias_symbol(first_source), :reflections=>{}, :reciprocals=>{}, :limits=>{}, :local=>opts, :cartesian_product_number=>0, :row_proc=>row_proc} 3276 ds = clone(:eager_graph=>opts) 3277 ds = ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations).naked 3278 end 3279 3280 ds.opts[:eager_graph].freeze 3281 ds.opts[:eager_graph].each_value{|v| v.freeze if v.is_a?(Hash)} 3282 ds 3283 end
If the dataset is being eagerly loaded, default to calling all instead of each.
# File lib/sequel/model/associations.rb 3297 def to_hash_groups(key_column, value_column=nil, opts=OPTS) 3298 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3299 opts = Hash[opts] 3300 opts[:all] = true 3301 end 3302 super 3303 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 3309 def ungraphed 3310 ds = super.clone(:eager_graph=>nil) 3311 if (eg = @opts[:eager_graph]) && (rp = eg[:row_proc]) 3312 ds = ds.with_row_proc(rp) 3313 end 3314 ds 3315 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 3334 def eager_graph_association(ds, model, ta, requirements, r, *associations) 3335 if r.is_a?(SQL::AliasedExpression) 3336 alias_base = r.alias 3337 if r.columns.is_a?(Hash) 3338 join_type = r.columns[:join_type] 3339 end 3340 r = r.expression 3341 else 3342 alias_base = r[:graph_alias_base] 3343 end 3344 assoc_table_alias = ds.unused_table_alias(alias_base) 3345 loader = r[:eager_grapher] 3346 if !associations.empty? 3347 if associations.first.respond_to?(:call) 3348 callback = associations.first 3349 associations = {} 3350 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) 3351 callback, assoc = pr_assoc 3352 associations = assoc.is_a?(Array) ? assoc : [assoc] 3353 end 3354 end 3355 local_opts = ds.opts[:eager_graph][:local] 3356 limit_strategy = r.eager_graph_limit_strategy(local_opts[:limit_strategy]) 3357 3358 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) 3359 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]}" 3360 end 3361 3362 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]) 3363 if r[:order_eager_graph] && (order = r.fetch(:graph_order, r[:order])) 3364 ds = ds.order_append(*qualified_expression(order, assoc_table_alias)) 3365 end 3366 eager_graph = ds.opts[:eager_graph] 3367 eager_graph[:requirements][assoc_table_alias] = requirements.dup 3368 eager_graph[:reflections][assoc_table_alias] = r 3369 if limit_strategy == :ruby 3370 eager_graph[:limits][assoc_table_alias] = r.limit_and_offset 3371 end 3372 eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2 3373 ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty? 3374 ds 3375 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 3386 def eager_graph_associations(ds, model, ta, requirements, *associations) 3387 associations.flatten.each do |association| 3388 ds = case association 3389 when Symbol, SQL::AliasedExpression 3390 ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, association)) 3391 when Hash 3392 association.each do |assoc, assoc_assocs| 3393 ds = ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, assoc), assoc_assocs) 3394 end 3395 ds 3396 else 3397 raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 3398 end 3399 end 3400 ds 3401 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 3405 def eager_graph_build_associations(hashes) 3406 hashes.replace(_eager_graph_build_associations(hashes, eager_graph_loader)) 3407 end