FIX: Multiple topics may have the same post as its solution (#348)

We are seeing some errors when migrating and adding indexes on `answer_post_id`.

```
#<StandardError:"An error has occurred, all later migrations canceled:\n\nPG::UniqueViolation: ERROR:  could not create unique index \"index_discourse_solved_solved_topics_on_answer_post_id\"\nDETAIL:  Key (answer_post_id)=(13006) is duplicated.\n">
```

This PR modifies the earlier migration, and also adds one before the addition of indexes to remove duplicates.

db/migrate/20250318024953_copy_solved_topic_custom_field_to_discourse_solved_solved_topics.rb

@@ -23,23 +23,31 @@ class CopySolvedTopicCustomFieldToDiscourseSolvedSolvedTopics < ActiveRecord::Mi
           created_at,
           updated_at
         )
-        SELECT DISTINCT ON (tc.topic_id)
+        SELECT
           tc.topic_id,
-          CAST(tc.value AS INTEGER),
+          tc.answer_post_id,
-          CAST(tc2.value AS INTEGER),
+          tc.topic_timer_id,
-          COALESCE(ua.acting_user_id, -1),
+          tc.accepter_user_id,
           tc.created_at,
           tc.updated_at
+        FROM (
+          SELECT
+            tc.topic_id,
+            CAST(tc.value AS INTEGER) AS answer_post_id,
+            CAST(tc2.value AS INTEGER) AS topic_timer_id,
+            COALESCE(ua.acting_user_id, -1) AS accepter_user_id,
+            tc.created_at,
+            tc.updated_at,
+            ROW_NUMBER() OVER (PARTITION BY tc.topic_id ORDER BY tc.created_at ASC) AS rn_topic,
+            ROW_NUMBER() OVER (PARTITION BY CAST(tc.value AS INTEGER) ORDER BY tc.created_at ASC) AS rn_answer
           FROM topic_custom_fields tc
-        LEFT JOIN topic_custom_fields tc2
+          LEFT JOIN topic_custom_fields tc2 ON tc2.topic_id = tc.topic_id AND tc2.name = 'solved_auto_close_topic_timer_id'
-          ON tc2.topic_id = tc.topic_id
+          LEFT JOIN user_actions ua ON ua.target_topic_id = tc.topic_id AND ua.action_type = 15
-          AND tc2.name = 'solved_auto_close_topic_timer_id'
-        LEFT JOIN user_actions ua
-          ON ua.target_topic_id = tc.topic_id
-          AND ua.action_type = 15
           WHERE tc.name = 'accepted_answer_post_id'
             AND tc.id > :last_id
             AND tc.id <= :last_id + :batch_size
+        ) tc
+        WHERE tc.rn_topic = 1 AND tc.rn_answer = 1
         ON CONFLICT DO NOTHING
       SQL
 

db/migrate/20250318024954_remove_duplicates_from_discourse_solved_solved_topics.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class RemoveDuplicatesFromDiscourseSolvedSolvedTopics < ActiveRecord::Migration[7.2]
+  def up
+    # remove duplicates on answer_post_id based on earliest created_at
+    DB.exec(<<~SQL)
+      DELETE FROM discourse_solved_solved_topics
+      WHERE id NOT IN (
+        SELECT id FROM (
+          SELECT id, ROW_NUMBER() OVER (PARTITION BY answer_post_id ORDER BY created_at) as row_num
+          FROM discourse_solved_solved_topics
+        ) t WHERE row_num = 1
+      )
+    SQL
+
+    # remove duplicates on topic_id based on earliest created_at
+    DB.exec(<<~SQL)
+      DELETE FROM discourse_solved_solved_topics
+      WHERE id NOT IN (
+        SELECT id FROM (
+          SELECT id, ROW_NUMBER() OVER (PARTITION BY topic_id ORDER BY created_at) as row_num
+          FROM discourse_solved_solved_topics
+        ) t WHERE row_num = 1
+      )
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20250325074111_copy_remaining_solved_topic_custom_field_to_discourse_solved_solved_topics.rb

@@ -25,23 +25,31 @@ class CopyRemainingSolvedTopicCustomFieldToDiscourseSolvedSolvedTopics < ActiveR
           created_at,
           updated_at
         )
-        SELECT DISTINCT ON (tc.topic_id)
+        SELECT
           tc.topic_id,
-          CAST(tc.value AS INTEGER),
+          tc.answer_post_id,
-          CAST(tc2.value AS INTEGER),
+          tc.topic_timer_id,
-          COALESCE(ua.acting_user_id, -1),
+          tc.accepter_user_id,
           tc.created_at,
           tc.updated_at
+        FROM (
+          SELECT
+            tc.topic_id,
+            CAST(tc.value AS INTEGER) AS answer_post_id,
+            CAST(tc2.value AS INTEGER) AS topic_timer_id,
+            COALESCE(ua.acting_user_id, -1) AS accepter_user_id,
+            tc.created_at,
+            tc.updated_at,
+            ROW_NUMBER() OVER (PARTITION BY tc.topic_id ORDER BY tc.created_at ASC) AS rn_topic,
+            ROW_NUMBER() OVER (PARTITION BY CAST(tc.value AS INTEGER) ORDER BY tc.created_at ASC) AS rn_answer
           FROM topic_custom_fields tc
-        LEFT JOIN topic_custom_fields tc2
+          LEFT JOIN topic_custom_fields tc2 ON tc2.topic_id = tc.topic_id AND tc2.name = 'solved_auto_close_topic_timer_id'
-          ON tc2.topic_id = tc.topic_id
+          LEFT JOIN user_actions ua ON ua.target_topic_id = tc.topic_id AND ua.action_type = 15
-          AND tc2.name = 'solved_auto_close_topic_timer_id'
-        LEFT JOIN user_actions ua
-          ON ua.target_topic_id = tc.topic_id
-          AND ua.action_type = 15
           WHERE tc.name = 'accepted_answer_post_id'
             AND tc.id > :last_id
             AND tc.id <= :last_id + :batch_size
+        ) tc
+        WHERE tc.rn_topic = 1 AND tc.rn_answer = 1
         ON CONFLICT DO NOTHING
       SQL
 

spec/models/copy_solved_topic_custom_field_spec.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require_relative "../../db/migrate/20250318024953_copy_solved_topic_custom_field_to_discourse_solved_solved_topics"
+
+RSpec.describe CopySolvedTopicCustomFieldToDiscourseSolvedSolvedTopics, type: :migration do
+  let(:migration) { described_class.new }
+
+  describe "handling duplicates" do
+    it "ensures only unique topic_id and answer_post_id are inserted" do
+      topic = Fabricate(:topic)
+      topic1 = Fabricate(:topic)
+      post1 = Fabricate(:post, topic: topic)
+      TopicCustomField.create!(
+        topic_id: topic.id,
+        name: "accepted_answer_post_id",
+        value: post1.id.to_s,
+      )
+      # explicit duplicate
+      TopicCustomField.create!(
+        topic_id: topic1.id,
+        name: "accepted_answer_post_id",
+        value: post1.id.to_s,
+      )
+
+      second_topic = Fabricate(:topic)
+      post2 = Fabricate(:post, topic: second_topic)
+      TopicCustomField.create!(
+        topic_id: second_topic.id,
+        name: "accepted_answer_post_id",
+        value: post2.id.to_s,
+      )
+
+      migration.up
+      expected_count = DiscourseSolved::SolvedTopic.count
+
+      expect(expected_count).to eq(2)
+
+      expect(DiscourseSolved::SolvedTopic.where(topic_id: topic.id).count).to eq(1)
+      expect(DiscourseSolved::SolvedTopic.where(answer_post_id: post1.id).count).to eq(1)
+    end
+  end
+end

spec/models/remove_duplicates_from_discourse_solved_solved_topics_spec.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+require_relative "../../db/migrate/20250318024954_remove_duplicates_from_discourse_solved_solved_topics"
+
+RSpec.describe RemoveDuplicatesFromDiscourseSolvedSolvedTopics, type: :migration do
+  let(:migration) { described_class.new }
+
+  before do
+    # temp drop unique constraints to allow testing duplicate entries
+    ActiveRecord::Base.connection.execute(
+      "DROP INDEX IF EXISTS index_discourse_solved_solved_topics_on_topic_id;",
+    )
+    ActiveRecord::Base.connection.execute(
+      "DROP INDEX IF EXISTS index_discourse_solved_solved_topics_on_answer_post_id;",
+    )
+  end
+
+  after do
+    DiscourseSolved::SolvedTopic.delete_all
+
+    # reapply unique indexes
+    ActiveRecord::Base.connection.execute(
+      "CREATE UNIQUE INDEX index_discourse_solved_solved_topics_on_topic_id ON discourse_solved_solved_topics (topic_id);",
+    )
+    ActiveRecord::Base.connection.execute(
+      "CREATE UNIQUE INDEX index_discourse_solved_solved_topics_on_answer_post_id ON discourse_solved_solved_topics (answer_post_id);",
+    )
+  end
+
+  describe "removal of duplicate answer_post_ids" do
+    it "keeps only the earliest record for each answer_post_id" do
+      topic1 = Fabricate(:topic)
+      post1 = Fabricate(:post, topic: topic1)
+      topic2 = Fabricate(:topic)
+      post2 = Fabricate(:post, topic: topic2)
+
+      earlier = Fabricate(:solved_topic, topic: topic1, answer_post: post1, created_at: 2.days.ago)
+      Fabricate(:solved_topic, topic: topic1, answer_post: post1, created_at: 1.day.ago)
+      Fabricate(:solved_topic, topic: topic1, answer_post: post1, created_at: Date.today)
+      another = Fabricate(:solved_topic, topic: topic2, answer_post: post2, created_at: Date.today)
+
+      expect(DiscourseSolved::SolvedTopic.count).to eq(4)
+      migration.up
+
+      expect(DiscourseSolved::SolvedTopic.count).to eq(2)
+      expect(DiscourseSolved::SolvedTopic.pluck(:id, :answer_post_id)).to contain_exactly(
+        [earlier.id, post1.id],
+        [another.id, post2.id],
+      )
+    end
+  end
+end