diff --git a/app/models/mention.rb b/app/models/mention.rb
index 32c4cc235..2bbb09a0e 100644
--- a/app/models/mention.rb
+++ b/app/models/mention.rb
@@ -2,9 +2,7 @@ class Mention < ApplicationRecord
   belongs_to :user
   belongs_to :mentionable, polymorphic: true
 
-  validates :user_id, presence: true,
-                      uniqueness: { scope: %i[mentionable_id
-                                              mentionable_type] }
+  validates :user_id, presence: true, uniqueness: { scope: %i[mentionable_id mentionable_type] }
   validates :mentionable_id, presence: true
   validates :mentionable_type, presence: true
   validate :permission
diff --git a/db/migrate/20200521103848_add_unique_index_to_reactions_user_id.rb b/db/migrate/20200521103848_add_unique_index_to_reactions_user_id.rb
new file mode 100644
index 000000000..950c3a02b
--- /dev/null
+++ b/db/migrate/20200521103848_add_unique_index_to_reactions_user_id.rb
@@ -0,0 +1,13 @@
+class AddUniqueIndexToReactionsUserId < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index(
+      :reactions,
+      %i[user_id reactable_id reactable_type category],
+      unique: true,
+      algorithm: :concurrently,
+      name: :index_reactions_on_user_id_reactable_id_reactable_type_category
+    )
+  end
+end
diff --git a/db/migrate/20200521103911_add_unique_index_to_notification_subscriptions_user_id.rb b/db/migrate/20200521103911_add_unique_index_to_notification_subscriptions_user_id.rb
new file mode 100644
index 000000000..c766e4656
--- /dev/null
+++ b/db/migrate/20200521103911_add_unique_index_to_notification_subscriptions_user_id.rb
@@ -0,0 +1,13 @@
+class AddUniqueIndexToNotificationSubscriptionsUserId < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index(
+      :notification_subscriptions,
+      %i[user_id notifiable_type notifiable_id],
+      unique: true,
+      algorithm: :concurrently,
+      name: :idx_notification_subs_on_user_id_notifiable_type_notifiable_id
+    )
+  end
+end
diff --git a/db/migrate/20200521103935_add_unique_index_to_mentions_user_id.rb b/db/migrate/20200521103935_add_unique_index_to_mentions_user_id.rb
new file mode 100644
index 000000000..72addeea2
--- /dev/null
+++ b/db/migrate/20200521103935_add_unique_index_to_mentions_user_id.rb
@@ -0,0 +1,13 @@
+class AddUniqueIndexToMentionsUserId < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index(
+      :mentions,
+      %i[user_id mentionable_id mentionable_type],
+      unique: true,
+      algorithm: :concurrently,
+      name: :index_mentions_on_user_id_and_mentionable_id_mentionable_type
+    )
+  end
+end
diff --git a/db/migrate/20200521103952_add_unique_index_to_html_variants_name.rb b/db/migrate/20200521103952_add_unique_index_to_html_variants_name.rb
new file mode 100644
index 000000000..85ac2f93f
--- /dev/null
+++ b/db/migrate/20200521103952_add_unique_index_to_html_variants_name.rb
@@ -0,0 +1,7 @@
+class AddUniqueIndexToHtmlVariantsName < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index :html_variants, :name, unique: true, algorithm: :concurrently
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index ea2742dc2..38f99ab6c 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_05_20_092951) do
+ActiveRecord::Schema.define(version: 2020_05_21_103952) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -574,6 +574,7 @@ ActiveRecord::Schema.define(version: 2020_05_20_092951) do
     t.string "target_tag"
     t.datetime "updated_at", null: false
     t.integer "user_id"
+    t.index ["name"], name: "index_html_variants_on_name", unique: true
   end
 
   create_table "identities", id: :serial, force: :cascade do |t|
@@ -595,6 +596,7 @@ ActiveRecord::Schema.define(version: 2020_05_20_092951) do
     t.string "mentionable_type"
     t.datetime "updated_at", null: false
     t.integer "user_id"
+    t.index ["user_id", "mentionable_id", "mentionable_type"], name: "index_mentions_on_user_id_and_mentionable_id_mentionable_type", unique: true
   end
 
   create_table "messages", force: :cascade do |t|
@@ -628,6 +630,7 @@ ActiveRecord::Schema.define(version: 2020_05_20_092951) do
     t.datetime "updated_at", null: false
     t.bigint "user_id", null: false
     t.index ["notifiable_id", "notifiable_type", "config"], name: "index_notification_subscriptions_on_notifiable_and_config"
+    t.index ["user_id", "notifiable_type", "notifiable_id"], name: "idx_notification_subs_on_user_id_notifiable_type_notifiable_id", unique: true
   end
 
   create_table "notifications", id: :serial, force: :cascade do |t|
@@ -927,6 +930,7 @@ ActiveRecord::Schema.define(version: 2020_05_20_092951) do
     t.index ["reactable_id", "reactable_type"], name: "index_reactions_on_reactable_id_and_reactable_type"
     t.index ["reactable_id"], name: "index_reactions_on_reactable_id"
     t.index ["reactable_type"], name: "index_reactions_on_reactable_type"
+    t.index ["user_id", "reactable_id", "reactable_type", "category"], name: "index_reactions_on_user_id_reactable_id_reactable_type_category", unique: true
     t.index ["user_id"], name: "index_reactions_on_user_id"
   end