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 3304 def as_hash(key_column=nil, value_column=nil, opts=OPTS) 3305 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3306 opts = Hash[opts] 3307 opts[:all] = true 3308 end 3309 super 3310 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 3010 def association_join(*associations) 3011 association_inner_join(*associations) 3012 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 3021 def complex_expression_sql_append(sql, op, args) 3022 r = args[1] 3023 if (((op == :'=' || op == :'!=') && r.is_a?(Sequel::Model)) || 3024 (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)}))))) 3025 l = args[0] 3026 if ar = model.association_reflections[l] 3027 raise Error, "filtering by associations is not allowed for #{ar.inspect}" if ar[:allow_filtering_by] == false 3028 3029 if multiple 3030 klass = ar.associated_class 3031 if is_ds 3032 if r.respond_to?(:model) 3033 unless r.model <= klass 3034 # A dataset for a different model class, could be a valid regular query 3035 return super 3036 end 3037 else 3038 # Not a model dataset, could be a valid regular query 3039 return super 3040 end 3041 else 3042 unless r.all?{|x| x.is_a?(klass)} 3043 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}" 3044 end 3045 end 3046 elsif !r.is_a?(ar.associated_class) 3047 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}" 3048 end 3049 3050 if exp = association_filter_expression(op, ar, r) 3051 literal_append(sql, exp) 3052 else 3053 raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}" 3054 end 3055 elsif multiple && (is_ds || r.empty?) 3056 # Not a query designed for this support, could be a valid regular query 3057 super 3058 else 3059 raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}" 3060 end 3061 else 3062 super 3063 end 3064 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 3141 def eager(*associations) 3142 opts = @opts[:eager] 3143 association_opts = eager_options_for_associations(associations) 3144 opts = opts ? opts.merge(association_opts) : association_opts 3145 clone(:eager=>opts.freeze) 3146 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 3251 def eager_graph(*associations) 3252 eager_graph_with_options(associations) 3253 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 3275 def eager_graph_with_options(associations, opts=OPTS) 3276 return self if associations.empty? 3277 3278 opts = opts.dup unless opts.frozen? 3279 associations = [associations] unless associations.is_a?(Array) 3280 ds = if eg = @opts[:eager_graph] 3281 eg = eg.dup 3282 [:requirements, :reflections, :reciprocals, :limits].each{|k| eg[k] = eg[k].dup} 3283 eg[:local] = opts 3284 ds = clone(:eager_graph=>eg) 3285 ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations) 3286 else 3287 # Each of the following have a symbol key for the table alias, with the following values: 3288 # :reciprocals :: the reciprocal value to use for this association 3289 # :reflections :: AssociationReflection instance related to this association 3290 # :requirements :: array of requirements for this association 3291 # :limits :: Any limit/offset array slicing that need to be handled in ruby land after loading 3292 opts = {:requirements=>{}, :master=>alias_symbol(first_source), :reflections=>{}, :reciprocals=>{}, :limits=>{}, :local=>opts, :cartesian_product_number=>0, :row_proc=>row_proc} 3293 ds = clone(:eager_graph=>opts) 3294 ds = ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations).naked 3295 end 3296 3297 ds.opts[:eager_graph].freeze 3298 ds.opts[:eager_graph].each_value{|v| v.freeze if v.is_a?(Hash)} 3299 ds 3300 end
If the dataset is being eagerly loaded, default to calling all instead of each.
# File lib/sequel/model/associations.rb 3314 def to_hash_groups(key_column, value_column=nil, opts=OPTS) 3315 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3316 opts = Hash[opts] 3317 opts[:all] = true 3318 end 3319 super 3320 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 3326 def ungraphed 3327 ds = super.clone(:eager_graph=>nil) 3328 if (eg = @opts[:eager_graph]) && (rp = eg[:row_proc]) 3329 ds = ds.with_row_proc(rp) 3330 end 3331 ds 3332 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 3351 def eager_graph_association(ds, model, ta, requirements, r, *associations) 3352 if r.is_a?(SQL::AliasedExpression) 3353 alias_base = r.alias 3354 if r.columns.is_a?(Hash) 3355 join_type = r.columns[:join_type] 3356 end 3357 r = r.expression 3358 else 3359 alias_base = r[:graph_alias_base] 3360 end 3361 assoc_table_alias = ds.unused_table_alias(alias_base) 3362 loader = r[:eager_grapher] 3363 if !associations.empty? 3364 if associations.first.respond_to?(:call) 3365 callback = associations.first 3366 associations = {} 3367 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) 3368 callback, assoc = pr_assoc 3369 associations = assoc.is_a?(Array) ? assoc : [assoc] 3370 end 3371 end 3372 local_opts = ds.opts[:eager_graph][:local] 3373 limit_strategy = r.eager_graph_limit_strategy(local_opts[:limit_strategy]) 3374 3375 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) 3376 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]}" 3377 end 3378 3379 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]) 3380 if r[:order_eager_graph] && (order = r.fetch(:graph_order, r[:order])) 3381 ds = ds.order_append(*qualified_expression(order, assoc_table_alias)) 3382 end 3383 eager_graph = ds.opts[:eager_graph] 3384 eager_graph[:requirements][assoc_table_alias] = requirements.dup 3385 eager_graph[:reflections][assoc_table_alias] = r 3386 if limit_strategy == :ruby 3387 eager_graph[:limits][assoc_table_alias] = r.limit_and_offset 3388 end 3389 eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2 3390 ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty? 3391 ds 3392 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 3403 def eager_graph_associations(ds, model, ta, requirements, *associations) 3404 associations.flatten.each do |association| 3405 ds = case association 3406 when Symbol, SQL::AliasedExpression 3407 ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, association)) 3408 when Hash 3409 association.each do |assoc, assoc_assocs| 3410 ds = ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, assoc), assoc_assocs) 3411 end 3412 ds 3413 else 3414 raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 3415 end 3416 end 3417 ds 3418 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 3422 def eager_graph_build_associations(hashes) 3423 hashes.replace(_eager_graph_build_associations(hashes, eager_graph_loader)) 3424 end