backend/app/controllers/post_versions_controller.rb

@@ -30,7 +30,9 @@ class PostVersionsController < ApplicationController
     q = q.where('post_versions.post_id = ?', post_id) if post_id
     if tag_name
       escaped = ActiveRecord::Base.sanitize_sql_like(tag_name.name)
-      q = q.where("CONCAT(' ', post_versions.tags, ' ') LIKE ?", "% #{ escaped } %")
+      q = q.where(("CONCAT(' ', post_versions.tags, ' ') LIKE :kw " +
+                   "OR CONCAT(' ', prev.tags, ' ') LIKE :kw"),
+                  kw: "% #{ escaped } %")
     end
 
     count = q.except(:select, :order, :limit, :offset).count

backend/spec/requests/posts_spec.rb

@@ -927,16 +927,23 @@ RSpec.describe 'Posts API', type: :request do
       )
     end
 
-    it 'matches tag filter against current tags snapshot only' do
+    it 'filters versions by tag when the tag exists in either current or previous snapshot' do
       get '/posts/versions', params: { post: post_record.id, tag: tag.id }
 
       expect(response).to have_http_status(:ok)
-      expect(json.fetch('count')).to eq(1)
+      expect(json.fetch('count')).to eq(2)
 
       versions = json.fetch('versions')
-      expect(versions.size).to eq(1)
-      expect(versions[0]['version_no']).to eq(1)
-      expect(versions[0]['tags']).to include(
+      expect(versions.map { |v| v['post_id'] }).to all(eq(post_record.id))
+      expect(versions.map { |v| v['version_no'] }).to eq([2, 1])
+
+      latest = versions[0]
+      first = versions[1]
+
+      expect(latest['tags']).to include(
+        { 'name' => 'spec_tag', 'type' => 'removed' }
+      )
+      expect(first['tags']).to include(
         { 'name' => 'spec_tag', 'type' => 'added' }
       )
     end