diff --git a/backend/app/models/post_similarity.rb b/backend/app/models/post_similarity.rb
index a753518..0f86550 100644
--- a/backend/app/models/post_similarity.rb
+++ b/backend/app/models/post_similarity.rb
@@ -1,4 +1,6 @@
 class PostSimilarity < ApplicationRecord
+  self.primary_key = :post_id, :target_post_id
+
   belongs_to :post, class_name: 'Post', foreign_key: 'post_id'
   belongs_to :target_post, class_name: 'Post', foreign_key: 'target_post_id'
 end
diff --git a/backend/app/models/tag_similarity.rb b/backend/app/models/tag_similarity.rb
index d983409..7a6d252 100644
--- a/backend/app/models/tag_similarity.rb
+++ b/backend/app/models/tag_similarity.rb
@@ -1,4 +1,6 @@
 class TagSimilarity < ApplicationRecord
+  self.primary_key = :tag_id, :target_tag_id
+
   belongs_to :tag, class_name: 'Tag', foreign_key: 'tag_id'
   belongs_to :target_tag, class_name: 'Tag', foreign_key: 'target_tag_id'
 end
diff --git a/backend/db/migrate/20260121024000_change_post_similarities_to_composite_pk.rb b/backend/db/migrate/20260121024000_change_post_similarities_to_composite_pk.rb
new file mode 100644
index 0000000..1b86e30
--- /dev/null
+++ b/backend/db/migrate/20260121024000_change_post_similarities_to_composite_pk.rb
@@ -0,0 +1,43 @@
+class ChangePostSimilaritiesToCompositePk < ActiveRecord::Migration[8.0]
+  def up
+    execute <<~SQL
+      ALTER TABLE
+        post_similarities
+          MODIFY COLUMN id BIGINT NOT NULL
+      ;
+    SQL
+
+    execute <<~SQL
+      ALTER TABLE
+        post_similarities
+          DROP PRIMARY KEY
+      ;
+    SQL
+
+    remove_column :post_similarities, :id
+
+    execute <<~SQL
+      ALTER TABLE
+        post_similarities
+          ADD PRIMARY KEY (post_id, target_post_id)
+      ;
+    SQL
+  end
+
+  def down
+    execute <<~SQL
+      ALTER TABLE
+        post_similarities
+          DROP PRIMARY KEY
+      ;
+    SQL
+
+    execute <<~SQL
+      ALTER TABLE
+        post_similarities
+          ADD COLUMN id BIGINT NOT NULL AUTO_INCREMENT FIRST
+        , ADD PRIMARY KEY (id)
+      ;
+    SQL
+  end
+end
diff --git a/backend/db/migrate/20260121024800_change_tag_similarities_to_composite_pk.rb b/backend/db/migrate/20260121024800_change_tag_similarities_to_composite_pk.rb
new file mode 100644
index 0000000..7a8bfc0
--- /dev/null
+++ b/backend/db/migrate/20260121024800_change_tag_similarities_to_composite_pk.rb
@@ -0,0 +1,43 @@
+class ChangeTagSimilaritiesToCompositePk < ActiveRecord::Migration[8.0]
+  def up
+    execute <<~SQL
+      ALTER TABLE
+        tag_similarities
+          MODIFY COLUMN id BIGINT NOT NULL
+      ;
+    SQL
+
+    execute <<~SQL
+      ALTER TABLE
+        tag_similarities
+          DROP PRIMARY KEY
+      ;
+    SQL
+
+    remove_column :tag_similarities, :id
+
+    execute <<~SQL
+      ALTER TABLE
+        tag_similarities
+          ADD PRIMARY KEY (tag_id, target_tag_id)
+      ;
+    SQL
+  end
+
+  def down
+    execute <<~SQL
+      ALTER TABLE
+        tag_similarities
+          DROP PRIMARY KEY
+      ;
+    SQL
+
+    execute <<~SQL
+      ALTER TABLE
+        tag_similarities
+          ADD COLUMN id BIGINT NOT NULL AUTO_INCREMENT FIRST
+        , ADD PRIMARY KEY (id)
+      ;
+    SQL
+  end
+end
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index a67d0dd..ee14689 100644
--- a/backend/db/schema.rb
+++ b/backend/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_01_18_144400) do
+ActiveRecord::Schema[8.0].define(version: 2026_01_21_024800) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -55,7 +55,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_01_18_144400) do
     t.index ["tag_id"], name: "index_nico_tag_relations_on_tag_id"
   end
 
-  create_table "post_similarities", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+  create_table "post_similarities", primary_key: ["post_id", "target_post_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "post_id", null: false
     t.bigint "target_post_id", null: false
     t.float "cos", null: false
@@ -126,7 +126,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_01_18_144400) do
     t.index ["name"], name: "index_tag_names_on_name", unique: true
   end
 
-  create_table "tag_similarities", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+  create_table "tag_similarities", primary_key: ["tag_id", "target_tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "tag_id", null: false
     t.bigint "target_tag_id", null: false
     t.float "cos", null: false