FEATURE: tag group options: limit usage of one tag per group, tags in a group can't be used unless a prerequisite tag is used

lib/discourse_tagging.rb

@@ -33,7 +33,7 @@ module DiscourseTagging
 
       if tag_names.present?
         category = topic.category
-        tags = filter_allowed_tags(Tag.where(name: tag_names), guardian, { for_input: true, category: category }).to_a
+        tags = filter_allowed_tags(Tag.where(name: tag_names), guardian, { for_input: true, category: category, selected_tags: tag_names }).to_a
 
         if tags.size < tag_names.size && (category.nil? || category.tags.count == 0)
           tag_names.each do |name|
@@ -56,8 +56,9 @@ module DiscourseTagging
 
   # Options:
   #   term: a search term to filter tags by name
-  #   for_input: result is for an input field, so only show permitted tags
   #   category: a Category to which the object being tagged belongs
+  #   for_input: result is for an input field, so only show permitted tags
+  #   selected_tags: an array of tag names that are in the current selection
   def self.filter_allowed_tags(query, guardian, opts={})
     term = opts[:term]
     if term.present?
@@ -67,6 +68,8 @@ module DiscourseTagging
     end
 
     if opts[:for_input]
+      selected_tag_ids = opts[:selected_tags] ? Tag.where(name: opts[:selected_tags]).pluck(:id) : []
+
       unless guardian.is_staff?
         staff_tag_names = SiteSetting.staff_tags.split("|")
         query = query.where('tags.name NOT IN (?)', staff_tag_names) if staff_tag_names.present?
@@ -74,20 +77,24 @@ module DiscourseTagging
 
       # Filters for category-specific tags:
 
-      if opts[:category] && (opts[:category].tags.count > 0 || opts[:category].tag_groups.count > 0)
-        if opts[:category].tags.count > 0 && opts[:category].tag_groups.count > 0
-          tag_group_ids = opts[:category].tag_groups.pluck(:id)
+      category = opts[:category]
+
+      if category && (category.tags.count > 0 || category.tag_groups.count > 0)
+        if category.tags.count > 0 && category.tag_groups.count > 0
+          tag_group_ids = category.tag_groups.pluck(:id)
+
           query = query.where(
             "tags.id IN (SELECT tag_id FROM category_tags WHERE category_id = ?
               UNION
-              SELECT tag_id FROM tag_group_memberships WHERE tag_group_id = ?)",
-            opts[:category].id, tag_group_ids
+              SELECT tag_id FROM tag_group_memberships WHERE tag_group_id IN (?))",
+            category.id, tag_group_ids
           )
-        elsif opts[:category].tags.count > 0
-          query = query.where("tags.id IN (SELECT tag_id FROM category_tags WHERE category_id = ?)", opts[:category].id)
-        else # opts[:category].tag_groups.count > 0
-          tag_group_ids = opts[:category].tag_groups.pluck(:id)
-          query = query.where("tags.id IN (SELECT tag_id FROM tag_group_memberships WHERE tag_group_id = ?)", tag_group_ids)
+        elsif category.tags.count > 0
+          query = query.where("tags.id IN (SELECT tag_id FROM category_tags WHERE category_id = ?)", category.id)
+        else # category.tag_groups.count > 0
+          tag_group_ids = category.tag_groups.pluck(:id)
+
+          query = query.where("tags.id IN (SELECT tag_id FROM tag_group_memberships WHERE tag_group_id IN (?))", tag_group_ids)
         end
       else
         # exclude tags that are restricted to other categories
@@ -97,7 +104,41 @@ module DiscourseTagging
 
         if CategoryTagGroup.exists?
           tag_group_ids = CategoryTagGroup.pluck(:tag_group_id).uniq
-          query = query.where("tags.id NOT IN (SELECT tag_id FROM tag_group_memberships WHERE tag_group_id = ?)", tag_group_ids)
+          query = query.where("tags.id NOT IN (SELECT tag_id FROM tag_group_memberships WHERE tag_group_id IN (?))", tag_group_ids)
+        end
+      end
+
+      # exclude tag groups that have a parent tag which is missing from selected_tags
+
+      select_sql = <<-SQL
+      SELECT tag_id
+            FROM tag_group_memberships tgm
+      INNER JOIN tag_groups tg
+              ON tgm.tag_group_id = tg.id
+      SQL
+
+      if selected_tag_ids.empty?
+        sql = "tags.id NOT IN (#{select_sql} WHERE tg.parent_tag_id IS NOT NULL)"
+        query = query.where(sql)
+      else
+        # One tag per group restriction
+        exclude_group_ids = TagGroup.where(one_per_topic: true)
+                                    .joins(:tag_group_memberships)
+                                    .where('tag_group_memberships.tag_id in (?)', selected_tag_ids)
+                                    .pluck(:id)
+
+        if exclude_group_ids.empty?
+          sql = "tags.id NOT IN (#{select_sql} WHERE tg.parent_tag_id NOT IN (?))"
+          query = query.where(sql, selected_tag_ids)
+        else
+          # It's possible that the selected tags violate some one-tag-per-group restrictions,
+          # so filter them out by picking one from each group.
+          limit_tag_ids = TagGroupMembership.select('distinct on (tag_group_id) tag_id')
+                                            .where(tag_id: selected_tag_ids)
+                                            .where(tag_group_id: exclude_group_ids)
+                                            .map(&:tag_id)
+          sql = "(tags.id NOT IN (#{select_sql} WHERE (tg.parent_tag_id NOT IN (?) OR tg.id in (?))) OR tags.id IN (?))"
+          query = query.where(sql, selected_tag_ids, exclude_group_ids, limit_tag_ids)
         end
       end
     end