#434

backend/app/services/counts_axes.rb

@@ -0,0 +1,98 @@
+class CountsAxes
+  GRAINS = ['year', 'quarter', 'month', 'week', 'day', 'hour'].freeze
+
+  def self.time_expression(expression, grain, timezone)
+    raise ArgumentError unless GRAINS.include?(grain)
+
+    connection = ApplicationRecord.connection
+    # Use the installed TZInfo database, including DST, rather than MySQL timezone tables.
+    first = Time.utc(1900)
+    last = Time.utc(2101)
+    offset = timezone.period_for_utc(first).utc_total_offset
+    clauses = timezone.transitions_up_to(last, first).filter_map do |transition|
+      next if transition.offset.utc_total_offset == offset
+
+      previous = offset
+      offset = transition.offset.utc_total_offset
+      "WHEN #{ expression } < #{ connection.quote(transition.at.to_time) } THEN #{ previous }"
+    end
+    seconds = clauses.empty? ? offset.to_s : "CASE #{ clauses.join(' ') } ELSE #{ offset } END"
+    local = "DATE_ADD(#{ expression }, INTERVAL (#{ seconds }) SECOND)"
+    case grain
+    when 'year' then "DATE_FORMAT(#{ local }, '%Y')"
+    when 'quarter' then "CONCAT(YEAR(#{ local }), '-Q', QUARTER(#{ local }))"
+    when 'month' then "DATE_FORMAT(#{ local }, '%Y-%m')"
+    when 'week' then "DATE_FORMAT(#{ local }, '%x-W%v')"
+    when 'day' then "DATE_FORMAT(#{ local }, '%Y-%m-%d')"
+    else "DATE_FORMAT(#{ local }, '%Y-%m-%d %H:00')"
+    end
+  end
+
+  attr_reader :rows, :columns, :all
+
+  def initialize(population)
+    @population = population
+    @definition = population.definition
+    @rows = parse(population.params.fetch('rows', '[]'))
+    @columns = parse(population.params.fetch('cols', '[]'))
+    @all = rows + columns
+    raise ArgumentError if all.length > 8 || all.map { _1['field'] }.uniq.length != all.length
+  end
+
+  def expressions
+    all.map do |axis|
+      field = axis.fetch('field')
+      case field
+      when 'tags'
+        "CASE WHEN tag_entry.tag_id IS NOT NULL " +
+            "THEN CONCAT('internal:', tag_entry.tag_id) " +
+            "ELSE CONCAT('external:', tag_entry.external_tag_id) END"
+      when 'changed_field' then 'changed.field_key'
+      else
+        expression = @definition.expression(field)
+        @definition.fields[field] == :datetime ? self.class.time_expression(
+            expression, axis.fetch('grain', 'month'), @population.timezone) : expression
+      end
+    end
+  end
+
+  def joins
+    result = ''
+    if all.any? { _1['field'] == 'tags' }
+      result += <<~SQL.squish
+         JOIN JSON_TABLE(CAST(v.tags_json AS JSON), '$[*]' COLUMNS (
+          tag_id BIGINT PATH '$.tag_id' NULL ON EMPTY,
+          external_tag_id BIGINT PATH '$.external_tag_id' NULL ON EMPTY)) tag_entry
+
+      SQL
+    end
+    if all.any? { _1['field'] == 'changed_field' }
+      fields = @definition.changes.map do |field|
+        "SELECT #{ ApplicationRecord.connection.quote(field) } AS field_key"
+      end.join(' UNION ALL ')
+      condition = @definition.changes.map do |field|
+        "(changed.field_key = '#{ field }' AND #{ @population.changed_sql(field) })"
+      end.join(' OR ')
+      result += " JOIN (#{ fields }) changed ON (#{ condition })"
+    end
+    result
+  end
+
+  private
+
+  def parse(value)
+    axes = JSON.parse(value)
+    raise ArgumentError unless axes.is_a?(Array)
+
+    axes.each do |axis|
+      raise ArgumentError unless axis.is_a?(Hash) && (axis.keys - ['field', 'grain']).empty?
+      raise ArgumentError unless @definition.axes(@population.history).include?(axis['field'])
+      if @definition.fields[axis['field']] == :datetime
+        raise ArgumentError unless GRAINS.include?(axis.fetch('grain', 'month'))
+      elsif axis.key?('grain')
+        raise ArgumentError
+      end
+    end
+    axes
+  end
+end