pcreux · July 23, 2013 17:25 · Jul 23, 2013
diff --git a/cancan_use_subqueries.rb b/cancan_use_subqueries.rb
@@ -0,0 +1,80 @@
+require 'cancan/model_adapters/active_record_adapter'
+
+class CanCan::ModelAdapters::ActiveRecordAdapter
+  # From CanCan
+  def database_records
+    if override_scope
+      @model_class.scoped.merge(override_scope)
+    elsif @model_class.respond_to?(:where) && @model_class.respond_to?(:joins)
+      mergeable_conditions = @rules.select {|rule| rule.unmergeable? }.blank?
+      if mergeable_conditions
+        # TODO Only use nested queries when necessary
+        # Let's use nested queries
+        BuildNestedRelation.call(@model_class, @rules)
+      else
+        @model_class.where(*(@rules.map(&:conditions))).joins(joins)
+      end
+    else
+      @model_class.scoped(:conditions => conditions, :joins => joins)
+    end
+  end
+
+  class BuildNestedRelation
+    def self.call(*args)
+      new(*args).call
+    end
+
+    def initialize(model_class, rules)
+      self.model_class = model_class
+      self.rules = rules
+    end
+
+    def call
+      model_class.where(query)
+    end
+
+    private
+
+    # @return [String] like:
+    #   id IN ( SUBQUERY_FOR_RULE_1 ) OR id IN ( SUBQUERY_FOR_RULE_2 ) ...
+    #
+    def query
+      rules.map do |rule|
+        "#{select_id} IN (
+  #{subquery(rule).to_sql}
+)"
+      end.join("\nOR\n")
+    end
+
+    def select_id
+      %|"#{model_class.table_name}"."id"|
+    end
+
+    # @return [ActiveRecord::Relation] a subquery for the rule passed in
+    def subquery(rule)
+      model_class.
+        select(select_id).
+        joins(joins(rule)).
+        where(conditions(rule)).
+        reorder(nil). # work around ambiguous column error
+        limit(nil). # don't take limit into account
+        offset(nil) # don't take offset into account
+    end
+
+    # @return conditions statement for the rule passed in
+    def conditions(rule)
+      cancan_adapter(rule).conditions
+    end
+
+    # @return joins statement for the rule passed in
+    def joins(rule)
+      cancan_adapter(rule).joins
+    end
+
+    def cancan_adapter(rule)
+      CanCan::ModelAdapters::ActiveRecordAdapter.new(model_class, [rule])
+    end
+
+    attr_accessor :model_class, :rules
+  end
+end
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