post queries: add #replace_aliases method.

app/logical/post_query.rb

@@ -47,5 +47,15 @@ class PostQuery
     select_metatags(*names).first&.value
   end
 
-  memoize :tags
+  # Return a new query AST, with aliased tags replaced with real tags.
+  def replace_aliases
+    ast.replace_tags(aliases)
+  end
+
+  # A hash mapping aliased tag names to real tag names.
+  def aliases
+    TagAlias.aliases_for(tag_names)
+  end
+
+  memoize :tags, :aliases
 end

app/logical/post_query/ast.rb

@@ -254,6 +254,20 @@ class PostQuery
         end
       end
 
+      # Replace tags according to a hash mapping old tag names to new tag names.
+      #
+      # @param replacements [Hash<String, String>] A hash mapping old tag names to new tag names.
+      # @return [AST] A new AST with the tags replaced.
+      def replace_tags(replacements)
+        rewrite do |node|
+          if node.tag? && replacements.has_key?(node.name)
+            node(:tag, replacements[node.name])
+          else
+            node
+          end
+        end
+      end
+
       # Call the block on the AST repeatedly until the output stops changing.
       #
       # `ast.repeat_until_unchanged(&:trim)` is like doing `ast.trim.trim.trim...`

app/models/tag_alias.rb

@@ -10,9 +10,9 @@ class TagAlias < TagRelationship
 
   scope :empty, -> { joins(:consequent_tag).merge(Tag.empty) }
 
-  def self.to_aliased(names)
-    names = Array(names).map(&:to_s)
-    return [] if names.empty?
+  # Given a list of tag names, return a hash mapping aliased names to real names.
+  def self.aliases_for(names)
+    return {} if names.empty?
 
     aliases = active.where(antecedent_name: names).map { |ta| [ta.antecedent_name, ta.consequent_name] }.to_h
 
@@ -22,7 +22,14 @@ class TagAlias < TagRelationship
       aliases[abbrev] = tag.name if tag.present?
     end
 
-    names.map { |name| aliases[name] || name }
+    aliases
+  end
+
+  # Given a list of tag names, return a new list with aliased tag names replaced by real tag names.
+  def self.to_aliased(names)
+    names = Array.wrap(names).map(&:to_s)
+    aliases = aliases_for(names)
+    names.map { |name| aliases.fetch(name, name) }
   end
 
   def process!