diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index b93c609..a6620fd 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -4,13 +4,19 @@ jobs:
   test:
     strategy:
       matrix:
-        ruby: ["3.0", "3.1", "3.2"]
+        ruby: ["3.3", "3.4", "4.0"]
         gemfile: [Gemfile]
         include:
-          - ruby: "3.2"
+          - ruby: "3.3"
+            gemfile: "gemfiles/rails-8.0.gemfile"
+          - ruby: "3.3"
+            gemfile: "gemfiles/rails-7.2.gemfile"
+          - ruby: "3.3"
+            gemfile: "gemfiles/rails-7.1.gemfile"
+          - ruby: "3.3"
+            gemfile: "gemfiles/rails-7.0.gemfile"
+          - ruby: "4.0"
             gemfile: "gemfiles/rails-7.0.gemfile"
-          - ruby: "3.2"
-            gemfile: "gemfiles/rails-6.1.gemfile"
 
     env:
       BUNDLE_GEMFILE: "${{ matrix.gemfile }}"
@@ -23,7 +29,3 @@ jobs:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true
     - run: bundle exec rake
-    - name: Send coverage
-      uses: paambaati/codeclimate-action@v5
-      env:
-        CC_TEST_REPORTER_ID: 921d046db793707b7d0c0d1305970a137fdf98fbc6ceeb6c355c44757040244f
diff --git a/.ruby-version b/.ruby-version
index be94e6f..7919852 100644
--- a/.ruby-version
+++ b/.ruby-version
@@ -1 +1 @@
-3.2.2
+4.0.9
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 03ed53c..9e4e3ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,15 @@
 ## Not released yet
 
+## 0.6.2
+
+- Fix `rake api_spec` raising `NoMethodError` on Rails 7.0 or later https://github.com/wantedly/open_api_annotator/pull/20
+- Fix `$ref` of `belongs_to` with `serializer:` option being an array like `#/components/schemas/["User"]` https://github.com/wantedly/open_api_annotator/pull/20
+- Skip `has_many` annotated with a non-array type (e.g. `type: User`) or an empty array with a warning, instead of raising an error or emitting a broken `$ref` https://github.com/wantedly/open_api_annotator/pull/20
+- Require open_api >= 0.6.3 https://github.com/wantedly/open_api_annotator/pull/20
+- Bump Ruby and Rails https://github.com/wantedly/open_api_annotator/pull/20
+  - Supported Rubies: 3.3, 3.4, 4.0
+  - Supported Rails: 7.0, 7.1, 7.2, 8.0, 8.1
+
 ## 0.6.1
 
 - Reflect ownership transfer https://github.com/wantedly/open_api_annotator/pull/18
diff --git a/README.md b/README.md
index 0f0b560..7ebbaff 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# OpenApiAnnotator [![Gem Version](https://badge.fury.io/rb/open_api_annotator.svg)](https://badge.fury.io/rb/open_api_annotator) [![Build Status](https://travis-ci.org/wantedly/open_api_annotator.svg?branch=master)](https://travis-ci.org/wantedly/open_api_annotator) [![Maintainability](https://api.codeclimate.com/v1/badges/8be7a273496459c62190/maintainability)](https://codeclimate.com/github/wantedly/open_api_annotator/maintainability) [![Test Coverage](https://api.codeclimate.com/v1/badges/8be7a273496459c62190/test_coverage)](https://codeclimate.com/github/wantedly/open_api_annotator/test_coverage)
+# OpenApiAnnotator [![Gem Version](https://badge.fury.io/rb/open_api_annotator.svg)](https://badge.fury.io/rb/open_api_annotator) [![Build Status](https://travis-ci.org/wantedly/open_api_annotator.svg?branch=master)](https://travis-ci.org/wantedly/open_api_annotator)
 
 OpenApiAnnotator realizes to generate OpenAPI spec by annotating to controllers and serializers.
 If you use ActiveModelSerializer, this is the best way to generate OpenAPI spec.
diff --git a/gemfiles/rails-6.1.gemfile b/gemfiles/rails-7.1.gemfile
similarity index 87%
rename from gemfiles/rails-6.1.gemfile
rename to gemfiles/rails-7.1.gemfile
index 5fec9e5..76058de 100644
--- a/gemfiles/rails-6.1.gemfile
+++ b/gemfiles/rails-7.1.gemfile
@@ -5,4 +5,4 @@ git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
 # Specify your gem's dependencies in open_api_annotator.gemspec
 gemspec path: '..'
 
-gem 'railties', '~> 6.1.0'
+gem 'railties', '~> 7.1.0'
diff --git a/gemfiles/rails-7.2.gemfile b/gemfiles/rails-7.2.gemfile
new file mode 100644
index 0000000..527e70e
--- /dev/null
+++ b/gemfiles/rails-7.2.gemfile
@@ -0,0 +1,8 @@
+source "https://rubygems.org"
+
+git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
+
+# Specify your gem's dependencies in open_api_annotator.gemspec
+gemspec path: '..'
+
+gem 'railties', '~> 7.2.0'
diff --git a/gemfiles/rails-8.0.gemfile b/gemfiles/rails-8.0.gemfile
new file mode 100644
index 0000000..e5e63e5
--- /dev/null
+++ b/gemfiles/rails-8.0.gemfile
@@ -0,0 +1,8 @@
+source "https://rubygems.org"
+
+git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
+
+# Specify your gem's dependencies in open_api_annotator.gemspec
+gemspec path: '..'
+
+gem 'railties', '~> 8.0.0'
diff --git a/lib/open_api_annotator/components_builder.rb b/lib/open_api_annotator/components_builder.rb
index c43a2ce..cbe020d 100644
--- a/lib/open_api_annotator/components_builder.rb
+++ b/lib/open_api_annotator/components_builder.rb
@@ -56,8 +56,12 @@ def build_has_many_association_properties(serializer)
       properties = {}
       serializer.open_api_has_many_associations.each do |association|
         next unless association.valid?
-        content = association.type.first
-        return unless content
+        # Calling `first` on a non-array type (e.g. an ActiveRecord model) would run a query
+        content = association.type.first if association.type.is_a?(Array)
+        unless content
+          warn "Skipped #{serializer.name}##{association.name}: type of has_many should be an array with one element, but got #{association.type}."
+          next
+        end
         content_name = content.try(:name) || content.to_s
         properties[association.name.to_sym] = OpenApi::Schema.new(
           type: "array",
diff --git a/lib/open_api_annotator/paths_builder.rb b/lib/open_api_annotator/paths_builder.rb
index 3508f3b..98cccd0 100644
--- a/lib/open_api_annotator/paths_builder.rb
+++ b/lib/open_api_annotator/paths_builder.rb
@@ -140,7 +140,7 @@ class RoutesFinder
     def find_all
       @routes ||= Rails.application.routes.routes.routes.map do |route|
         parameters = []
-        path = PathResolver.new.resolve(route.path.ast, parameters)
+        path = PathResolver.new.resolve(route.path.spec, parameters)
         controller = route.requirements[:controller]
         action = route.requirements[:action]
         Route.new(http_verb: route.verb, path: path, controller_name: controller, action_name: action, parameters: parameters)
diff --git a/lib/open_api_annotator/serializer_annotatable.rb b/lib/open_api_annotator/serializer_annotatable.rb
index be0b2c9..e2a5a4e 100644
--- a/lib/open_api_annotator/serializer_annotatable.rb
+++ b/lib/open_api_annotator/serializer_annotatable.rb
@@ -77,7 +77,7 @@ def open_api_belongs_to_associations
           reflection.is_a?(ActiveModel::Serializer::BelongsToReflection)
         }.map do |reflection|
           serializer_class = reflection.options[:serializer]
-          type = serializer_class ? [serializer_class.open_api_resource_name] : reflection.options[:type]
+          type = serializer_class ? serializer_class.open_api_resource_name : reflection.options[:type]
           Association.new(reflection.name.to_sym, type, reflection.options[:nullable])
         end
       end
diff --git a/lib/open_api_annotator/version.rb b/lib/open_api_annotator/version.rb
index 56574ab..f0e5f3b 100644
--- a/lib/open_api_annotator/version.rb
+++ b/lib/open_api_annotator/version.rb
@@ -1,3 +1,3 @@
 module OpenApiAnnotator
-  VERSION = "0.6.1"
+  VERSION = "0.6.2"
 end
diff --git a/open_api_annotator.gemspec b/open_api_annotator.gemspec
index 5420347..691855f 100644
--- a/open_api_annotator.gemspec
+++ b/open_api_annotator.gemspec
@@ -21,14 +21,13 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "open_api", ">= 0.5.0"
+  spec.add_dependency "open_api", ">= 0.6.3"
   spec.add_dependency "active_model_serializers", "~> 0.10.0"
 
-  rails_versions = ['>= 5.0', '< 7.2']
+  rails_versions = ['>= 5.0', '< 8.2']
   spec.add_dependency "actionpack", rails_versions
   spec.add_dependency "railties", rails_versions
 
-  spec.add_development_dependency "bundler", "~> 2.0"
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.0"
   spec.add_development_dependency "pry"
diff --git a/spec/open_api_annotator/components_builder_spec.rb b/spec/open_api_annotator/components_builder_spec.rb
index cfc6345..d0610d2 100644
--- a/spec/open_api_annotator/components_builder_spec.rb
+++ b/spec/open_api_annotator/components_builder_spec.rb
@@ -89,6 +89,35 @@
         )
       end
     end
+
+    context "when has_many associations are annotated with non-array types" do
+      let(:class_has_many_association) { OpenApiAnnotator::Association.new("logs", Log, true) }
+      let(:string_has_many_association) { OpenApiAnnotator::Association.new("notes", "Note", true) }
+
+      before do
+        # Like an ActiveRecord model, `first` would run a query
+        stub_const("Log", Class.new { def self.first; raise "Log.first must not be called"; end })
+        allow(BookSerializer).to receive(:open_api_has_many_associations).and_return([class_has_many_association, string_has_many_association, has_many_association])
+      end
+
+      it "skips only the associations with warnings" do
+        expect { subject }.to output(/Skipped BookSerializer#logs: .* but got Log\..*Skipped BookSerializer#notes: .* but got Note\./m).to_stderr
+        expect(subject.schemas["Book"].properties.keys).to contain_exactly(:id, :published_at, :tags, :authors, :cover_image, :publisher)
+      end
+    end
+
+    context "when a has_many association is annotated with an empty array" do
+      let(:empty_array_has_many_association) { OpenApiAnnotator::Association.new("logs", [], true) }
+
+      before do
+        allow(BookSerializer).to receive(:open_api_has_many_associations).and_return([empty_array_has_many_association, has_many_association])
+      end
+
+      it "skips only the association with a warning" do
+        expect { subject }.to output(/Skipped BookSerializer#logs/).to_stderr
+        expect(subject.schemas["Book"].properties.keys).to contain_exactly(:id, :published_at, :tags, :authors, :cover_image, :publisher)
+      end
+    end
   end
 
 end
diff --git a/spec/open_api_annotator/routes_finder_spec.rb b/spec/open_api_annotator/routes_finder_spec.rb
index f83eb47..bab5026 100644
--- a/spec/open_api_annotator/routes_finder_spec.rb
+++ b/spec/open_api_annotator/routes_finder_spec.rb
@@ -12,7 +12,7 @@
     let(:journey_route) { double(:journey_route) }
 
     before do
-      allow(path).to receive(:ast).and_return(double(:ast))
+      allow(path).to receive(:spec).and_return(double(:spec))
       allow(journey_route).to receive(:path).and_return(path)
       allow(journey_route).to receive(:verb).and_return("GET")
       allow(journey_route).to receive(:requirements).and_return({ controller: "/api/v1/books", action: "show" })
@@ -36,6 +36,40 @@
       )
     end
   end
+
+  describe "#find_all with routes drawn by Rails" do
+    subject { described_class.new.find_all }
+
+    let(:rails_application) { double(:rails_application) }
+    let(:route_set) do
+      ActionDispatch::Routing::RouteSet.new.tap do |route_set|
+        route_set.draw do
+          get "/api/v1/books/:book_id/authors/:id", to: "api/v1/authors#show"
+        end
+        # Rails 7.0+ wraps Pattern#ast in Journey::Ast and clears it on eager_load!, while Pattern#spec survives both
+        route_set.eager_load!
+      end
+    end
+
+    before do
+      allow(Rails).to receive(:application).and_return(rails_application)
+      allow(rails_application).to receive(:routes).and_return(route_set)
+    end
+
+    it "resolves path and parameters from the route" do
+      is_expected.to match(
+        [
+          OpenApiAnnotator::Route.new(
+            http_verb: "GET",
+            path: "/api/v1/books/{book_id}/authors/{id}",
+            controller_name: "api/v1/authors",
+            action_name: "show",
+            parameters: [{ name: "book_id" }, { name: "id" }]
+          )
+        ]
+      )
+    end
+  end
 end
 
 RSpec.describe OpenApiAnnotator::PathResolver do
diff --git a/spec/open_api_annotator/serializer_annotatable_spec.rb b/spec/open_api_annotator/serializer_annotatable_spec.rb
new file mode 100644
index 0000000..5310b0a
--- /dev/null
+++ b/spec/open_api_annotator/serializer_annotatable_spec.rb
@@ -0,0 +1,41 @@
+require 'spec_helper'
+require 'active_model_serializers'
+
+RSpec.describe OpenApiAnnotator::SerializerAnnotatable do
+  before do
+    # Annotations without `type:` are warned via Rails.logger
+    allow(Rails).to receive(:logger).and_return(Logger.new(nil))
+    stub_const("AuthorSerializer", Class.new(ActiveModel::Serializer) { include OpenApiAnnotator::SerializerAnnotatable })
+    stub_const("BookSerializer", Class.new(ActiveModel::Serializer) { include OpenApiAnnotator::SerializerAnnotatable })
+  end
+
+  describe ".open_api_has_many_associations" do
+    subject { BookSerializer.open_api_has_many_associations }
+
+    before { BookSerializer.has_many :authors, nullable: false, serializer: AuthorSerializer }
+
+    it "returns the resource name of the serializer as an array type" do
+      is_expected.to eq [OpenApiAnnotator::Association.new(:authors, ["Author"], false)]
+    end
+  end
+
+  describe ".open_api_has_one_associations" do
+    subject { BookSerializer.open_api_has_one_associations }
+
+    before { BookSerializer.has_one :author, nullable: false, serializer: AuthorSerializer }
+
+    it "returns the resource name of the serializer as a single type" do
+      is_expected.to eq [OpenApiAnnotator::Association.new(:author, "Author", false)]
+    end
+  end
+
+  describe ".open_api_belongs_to_associations" do
+    subject { BookSerializer.open_api_belongs_to_associations }
+
+    before { BookSerializer.belongs_to :author, nullable: false, serializer: AuthorSerializer }
+
+    it "returns the resource name of the serializer as a single type" do
+      is_expected.to eq [OpenApiAnnotator::Association.new(:author, "Author", false)]
+    end
+  end
+end