Fix error generating unique index in DB migration (#43)

Syntax for creating the index was incorrect

CHANGELOG.md

@@ -3,7 +3,7 @@ CHANGELOG
 
 - **Unreleased**
   * [View Diff](https://github.com/westonganger/active_snapshot/compare/v0.3.1...master)
-  * Nothing yet
+  * [#43](https://github.com/westonganger/active_snapshot/pull/43) - Fix unique index error in generated DB migration
 
 - **v0.3.1** - August 4, 2023
   * [View Diff](https://github.com/westonganger/active_snapshot/compare/v0.3.0...v0.3.1)

lib/generators/active_snapshot/install/templates/create_snapshots_tables.rb.erb

@@ -3,16 +3,22 @@ class <%= migration_name %> < ActiveRecord::Migration::Current
   def change
     create_table :snapshots<%= table_options %> do |t|
       t.belongs_to :item, polymorphic: true, null: false, index: true
-      t.string :identifier, unique: [:item_id, :item_type], index: true
       t.belongs_to :user, polymorphic: true
+
+      t.string :identifier, index: true
+      t.index [:identifier, :item_id, :item_type], unique: true
+
       t.<%= ActiveSnapshot.config.storage_method == 'native_json' ? 'json' : 'text' %> :metadata
+
       t.datetime :created_at, null: false
     end
 
     create_table :snapshot_items<%= table_options %> do |t|
       t.belongs_to :snapshot, null: false, index: true
       t.belongs_to :item, polymorphic: true, null: false, unique: [:snapshot_id], index: true
+
       t.<%= ActiveSnapshot.config.storage_method == 'native_json' ? 'json' : 'text' %> :object, null: false
+
       t.datetime :created_at, null: false
       t.string :child_group_name
     end

test/dummy_app/db/migrate/20210306100122_create_snapshots_tables.rb

@@ -3,14 +3,12 @@ class CreateSnapshotsTables < ActiveRecord::Migration::Current
   def change
     create_table :snapshots do |t|
       t.belongs_to :item, polymorphic: true, null: false, index: true
-      t.string :identifier, unique: true, index: true
       t.belongs_to :user, polymorphic: true
 
-      if ActiveSnapshot.config.storage_method_native_json?
-        t.json :metadata
-      else
-        t.text :metadata
-      end
+      t.string :identifier, index: true
+      t.index [:identifier, :item_id, :item_type], unique: true
+
+      t.<%= ActiveSnapshot.config.storage_method == 'native_json' ? 'json' : 'text' %> :metadata, null: false
 
       t.datetime :created_at, null: false
     end
@@ -19,11 +17,7 @@ def change
       t.belongs_to :snapshot, null: false, index: true
       t.belongs_to :item, polymorphic: true, null: false, unique: [:snapshot_id], index: true
 
-      if ActiveSnapshot.config.storage_method_native_json?
-        t.json :object, null: false
-      else
-        t.text :object, null: false
-      end
+      t.<%= ActiveSnapshot.config.storage_method == 'native_json' ? 'json' : 'text' %> :object, null: false
 
       t.datetime :created_at, null: false
       t.string :child_group_name