reports: add ability to group reports by column.

Add ability to group reports by various columns. For example, you can see
the posts by the top 10 uploaders over time, or posts grouped by rating
over time.

app/logical/danbooru/data_frame.rb

@@ -0,0 +1,47 @@
+# A wrapper around Rover::DataFrame that adds some extra utility methods.
+#
+# @see https://github.com/ankane/rover
+module Danbooru
+  class DataFrame
+    attr_reader :df
+    delegate :head, :shape, :types, :rename, :each_row, :[], :[]=, to: :df
+
+    def initialize(...)
+      @df = Rover::DataFrame.new(...)
+    end
+
+    # Replace ID columns with the actual object. For example, replace the `user_id` column with a `user` column containing User objects.
+    def preload_associations(associations)
+      associations.reduce(dup) do |table, association|
+        primary_key = association.association_primary_key
+        foreign_key = association.foreign_key
+        name = association.name.to_s
+
+        ids = table[foreign_key].to_a.uniq.compact_blank
+        records = association.klass.where(primary_key => ids).index_by(&primary_key.to_sym)
+
+        table.rename({ foreign_key => name })
+        table[name] = table[name].map { |id| records[id] }
+        table
+      end
+    end
+
+    def crosstab(index, pivot)
+      new_df = DataFrame.new(index => df[index].uniq)
+
+      df[pivot].uniq.to_a.each do |value|
+        columns = df.types.keys.without(index, pivot)
+        columns.each do |column|
+          name = columns.one? ? value.to_s : "#{value}_#{column}"
+          new_df[name] = df[df[pivot] == value][column]
+        end
+      end
+
+      new_df
+    end
+
+    def as_json(*options)
+      df.to_a
+    end
+  end
+end