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 3336 def as_hash(key_column=nil, value_column=nil, opts=OPTS) 3337 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3338 opts = Hash[opts] 3339 opts[:all] = true 3340 end 3341 super 3342 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 3042 def association_join(*associations) 3043 association_inner_join(*associations) 3044 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 3053 def complex_expression_sql_append(sql, op, args) 3054 r = args[1] 3055 if (((op == :'=' || op == :'!=') && r.is_a?(Sequel::Model)) || 3056 (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)}))))) 3057 l = args[0] 3058 if ar = model.association_reflections[l] 3059 raise Error, "filtering by associations is not allowed for #{ar.inspect}" if ar[:allow_filtering_by] == false 3060 3061 if multiple 3062 klass = ar.associated_class 3063 if is_ds 3064 if r.respond_to?(:model) 3065 unless r.model <= klass 3066 # A dataset for a different model class, could be a valid regular query 3067 return super 3068 end 3069 else 3070 # Not a model dataset, could be a valid regular query 3071 return super 3072 end 3073 else 3074 unless r.all?{|x| x.is_a?(klass)} 3075 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}" 3076 end 3077 end 3078 elsif !r.is_a?(ar.associated_class) 3079 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}" 3080 end 3081 3082 if exp = association_filter_expression(op, ar, r) 3083 literal_append(sql, exp) 3084 else 3085 raise Sequel::Error, "invalid association type #{ar[:type].inspect} for association #{l.inspect} used in dataset filter for model #{model.inspect}" 3086 end 3087 elsif multiple && (is_ds || r.empty?) 3088 # Not a query designed for this support, could be a valid regular query 3089 super 3090 else 3091 raise Sequel::Error, "invalid association #{l.inspect} used in dataset filter for model #{model.inspect}" 3092 end 3093 else 3094 super 3095 end 3096 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 3173 def eager(*associations) 3174 opts = @opts[:eager] 3175 association_opts = eager_options_for_associations(associations) 3176 opts = opts ? opts.merge(association_opts) : association_opts 3177 clone(:eager=>opts.freeze) 3178 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 3283 def eager_graph(*associations) 3284 eager_graph_with_options(associations) 3285 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 3307 def eager_graph_with_options(associations, opts=OPTS) 3308 return self if associations.empty? 3309 3310 opts = opts.dup unless opts.frozen? 3311 associations = [associations] unless associations.is_a?(Array) 3312 ds = if eg = @opts[:eager_graph] 3313 eg = eg.dup 3314 [:requirements, :reflections, :reciprocals, :limits].each{|k| eg[k] = eg[k].dup} 3315 eg[:local] = opts 3316 ds = clone(:eager_graph=>eg) 3317 ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations) 3318 else 3319 # Each of the following have a symbol key for the table alias, with the following values: 3320 # :reciprocals :: the reciprocal value to use for this association 3321 # :reflections :: AssociationReflection instance related to this association 3322 # :requirements :: array of requirements for this association 3323 # :limits :: Any limit/offset array slicing that need to be handled in ruby land after loading 3324 opts = {:requirements=>{}, :master=>alias_symbol(first_source), :reflections=>{}, :reciprocals=>{}, :limits=>{}, :local=>opts, :cartesian_product_number=>0, :row_proc=>row_proc} 3325 ds = clone(:eager_graph=>opts) 3326 ds = ds.eager_graph_associations(ds, model, ds.opts[:eager_graph][:master], [], *associations).naked 3327 end 3328 3329 ds.opts[:eager_graph].freeze 3330 ds.opts[:eager_graph].each_value{|v| v.freeze if v.is_a?(Hash)} 3331 ds 3332 end
If the dataset is being eagerly loaded, default to calling all instead of each.
# File lib/sequel/model/associations.rb 3346 def to_hash_groups(key_column, value_column=nil, opts=OPTS) 3347 if (@opts[:eager_graph] || @opts[:eager]) && !opts.has_key?(:all) 3348 opts = Hash[opts] 3349 opts[:all] = true 3350 end 3351 super 3352 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 3358 def ungraphed 3359 ds = super.clone(:eager_graph=>nil) 3360 if (eg = @opts[:eager_graph]) && (rp = eg[:row_proc]) 3361 ds = ds.with_row_proc(rp) 3362 end 3363 ds 3364 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 3383 def eager_graph_association(ds, model, ta, requirements, r, *associations) 3384 if r.is_a?(SQL::AliasedExpression) 3385 alias_base = r.alias 3386 if r.columns.is_a?(Hash) 3387 join_type = r.columns[:join_type] 3388 end 3389 r = r.expression 3390 else 3391 alias_base = r[:graph_alias_base] 3392 end 3393 assoc_table_alias = ds.unused_table_alias(alias_base) 3394 loader = r[:eager_grapher] 3395 if !associations.empty? 3396 if associations.first.respond_to?(:call) 3397 callback = associations.first 3398 associations = {} 3399 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) 3400 callback, assoc = pr_assoc 3401 associations = assoc.is_a?(Array) ? assoc : [assoc] 3402 end 3403 end 3404 local_opts = ds.opts[:eager_graph][:local] 3405 limit_strategy = r.eager_graph_limit_strategy(local_opts[:limit_strategy]) 3406 3407 # SEQUEL6: remove and integrate the auto_restrict_eager_graph plugin 3408 if !r[:orig_opts].has_key?(:graph_conditions) && !r[:orig_opts].has_key?(:graph_only_conditions) && !r.has_key?(:graph_block) && !r[:allow_eager_graph] 3409 if r[:conditions] && !Sequel.condition_specifier?(r[:conditions]) 3410 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]}" 3411 end 3412 3413 if r[:block] && !r[:graph_use_association_block] 3414 warn "eager_graph used for association when association given a block without graph options. The block is ignored in this case. This will result in an exception starting in Sequel 6. Model: #{r[:model]}, association: #{r[:name]}" 3415 end 3416 end 3417 3418 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]) 3419 if r[:order_eager_graph] && (order = r.fetch(:graph_order, r[:order])) 3420 ds = ds.order_append(*qualified_expression(order, assoc_table_alias)) 3421 end 3422 eager_graph = ds.opts[:eager_graph] 3423 eager_graph[:requirements][assoc_table_alias] = requirements.dup 3424 eager_graph[:reflections][assoc_table_alias] = r 3425 if limit_strategy == :ruby 3426 eager_graph[:limits][assoc_table_alias] = r.limit_and_offset 3427 end 3428 eager_graph[:cartesian_product_number] += r[:cartesian_product_number] || 2 3429 ds = ds.eager_graph_associations(ds, r.associated_class, assoc_table_alias, requirements + [assoc_table_alias], *associations) unless associations.empty? 3430 ds 3431 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 3442 def eager_graph_associations(ds, model, ta, requirements, *associations) 3443 associations.flatten.each do |association| 3444 ds = case association 3445 when Symbol, SQL::AliasedExpression 3446 ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, association)) 3447 when Hash 3448 association.each do |assoc, assoc_assocs| 3449 ds = ds.eager_graph_association(ds, model, ta, requirements, eager_graph_check_association(model, assoc), assoc_assocs) 3450 end 3451 ds 3452 else 3453 raise(Sequel::Error, 'Associations must be in the form of a symbol or hash') 3454 end 3455 end 3456 ds 3457 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 3461 def eager_graph_build_associations(hashes) 3462 hashes.replace(_eager_graph_build_associations(hashes, eager_graph_loader)) 3463 end