diff --git a/acceptance/cases/models/binary_identifiers.rb b/acceptance/cases/models/binary_identifiers.rb
new file mode 100644
index 00000000..94e859b2
--- /dev/null
+++ b/acceptance/cases/models/binary_identifiers.rb
@@ -0,0 +1,87 @@
+# Copyright 2025 Google LLC
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+
+# frozen_string_literal: true
+
+require "test_helper"
+require "test_helpers/with_separate_database"
+require_relative "../../models/user"
+require_relative "../../models/binary_project"
+
+module Models
+  class DefaultValueTest < SpannerAdapter::TestCase
+    include TestHelpers::WithSeparateDatabase
+
+    def setup
+      super
+
+      connection.create_table :users, id: :binary do |t|
+        t.string :email, null: false
+        t.string :full_name, null: false
+      end
+      connection.create_table :binary_projects, id: :binary do |t|
+        t.string :name, null: false
+        t.string :description, null: false
+        t.binary :owner_id, null: false
+        t.foreign_key :users, column: :owner_id
+      end
+    end
+
+    def test_includes_works
+      user = User.create!(
+        email: "test@example.com",
+        full_name: "Test User"
+      )
+      3.times do |i|
+        Project.create!(
+          name: "Project #{i}",
+          description: "Description #{i}",
+          owner: user
+        )
+      end
+
+      # First verify the association works without includes
+      projects = Project.all
+      assert_equal 3, projects.count
+
+      # Compare the base64 content instead of the StringIO objects
+      first_project = projects.first
+      assert_equal to_base64(user.id), to_base64(first_project.owner_id)
+
+      # Now verify includes is working
+      query_count = count_queries do
+        loaded_projects = Project.includes(:owner).to_a
+        loaded_projects.each do |project|
+          # Access the owner to ensure it's preloaded
+          assert_equal user.full_name, project.owner.full_name
+        end
+      end
+
+      # Spanner should execute 2 queries: one for projects and one for users
+      assert_equal 2, query_count
+    end
+
+    private
+
+    def to_base64 buffer
+      buffer.rewind
+      value = buffer.read
+      Base64.strict_encode64 value.force_encoding("ASCII-8BIT")
+    end
+
+    def count_queries(&block)
+      count = 0
+      counter_fn = ->(name, started, finished, unique_id, payload) {
+        unless %w[CACHE SCHEMA].include?(payload[:name])
+          count += 1
+        end
+      }
+
+      ActiveSupport::Notifications.subscribed(counter_fn, "sql.active_record", &block)
+      count
+    end
+  end
+end
diff --git a/acceptance/models/binary_project.rb b/acceptance/models/binary_project.rb
new file mode 100644
index 00000000..8e817b37
--- /dev/null
+++ b/acceptance/models/binary_project.rb
@@ -0,0 +1,18 @@
+# Copyright 2025 Google LLC
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+
+# frozen_string_literal: true
+
+class BinaryProject < ActiveRecord::Base
+  belongs_to :owner, class_name: 'User'
+
+  before_create :set_uuid
+  private
+
+  def set_uuid
+    self.id ||= StringIO.new(SecureRandom.random_bytes(16))
+  end
+end
diff --git a/acceptance/models/user.rb b/acceptance/models/user.rb
new file mode 100644
index 00000000..f5fbd330
--- /dev/null
+++ b/acceptance/models/user.rb
@@ -0,0 +1,18 @@
+# Copyright 2025 Google LLC
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+
+# frozen_string_literal: true
+
+class User < ActiveRecord::Base
+  has_many :binary_projects, foreign_key: :owner_id
+
+  before_create :set_uuid
+  private
+
+  def set_uuid
+    self.id ||= StringIO.new(SecureRandom.random_bytes(16))
+  end
+end
diff --git a/lib/active_record/type/spanner/bytes.rb b/lib/active_record/type/spanner/bytes.rb
index 1cee6452..2a07446d 100644
--- a/lib/active_record/type/spanner/bytes.rb
+++ b/lib/active_record/type/spanner/bytes.rb
@@ -10,6 +10,16 @@ module ActiveRecord
   module Type
     module Spanner
       class Bytes < ActiveRecord::Type::Binary
+        def deserialize value
+          # Set this environment variable to disable de-serializing BYTES
+          # to a StringIO instance.
+          return super if ENV["SPANNER_BYTES_DESERIALIZE_DISABLED"]
+
+          return super value if value.nil?
+          return StringIO.new Base64.strict_decode64(value) if value.is_a? ::String
+          value
+        end
+
         def serialize value
           return super value if value.nil?
 
diff --git a/test/activerecord_spanner_mock_server/base_spanner_mock_server_test.rb b/test/activerecord_spanner_mock_server/base_spanner_mock_server_test.rb
index 35325e07..3235fcf4 100644
--- a/test/activerecord_spanner_mock_server/base_spanner_mock_server_test.rb
+++ b/test/activerecord_spanner_mock_server/base_spanner_mock_server_test.rb
@@ -15,6 +15,8 @@
 require_relative "models/table_with_commit_timestamp"
 require_relative "models/table_with_sequence"
 require_relative "models/versioned_singer"
+require_relative "models/user"
+require_relative "models/binary_project"
 
 require "securerandom"
 
@@ -57,6 +59,15 @@ def setup
       MockServerTests::register_table_with_sequence_columns_result @mock
       MockServerTests::register_table_with_sequence_primary_key_columns_result @mock
       MockServerTests::register_table_with_sequence_primary_and_parent_key_columns_result @mock
+
+      MockServerTests::register_users_columns_result @mock
+      MockServerTests::register_users_primary_key_columns_result @mock
+      MockServerTests::register_users_primary_and_parent_key_columns_result @mock
+
+      MockServerTests::register_binary_projects_columns_result @mock
+      MockServerTests::register_binary_projects_primary_key_columns_result @mock
+      MockServerTests::register_binary_projects_primary_and_parent_key_columns_result @mock
+
       # Connect ActiveRecord to the mock server
       ActiveRecord::Base.establish_connection(
         adapter: "spanner",
diff --git a/test/activerecord_spanner_mock_server/model_helper.rb b/test/activerecord_spanner_mock_server/model_helper.rb
index bcbd28bb..28617f14 100644
--- a/test/activerecord_spanner_mock_server/model_helper.rb
+++ b/test/activerecord_spanner_mock_server/model_helper.rb
@@ -192,6 +192,24 @@ def self.register_select_tables_result spanner_mock_server
       Value.new(null_value: "NULL_VALUE"),
     )
     result_set.rows.push row
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: ""),
+      Value.new(string_value: ""),
+      Value.new(string_value: "users"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+    )
+    result_set.rows.push row
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: ""),
+      Value.new(string_value: ""),
+      Value.new(string_value: "binary_projects"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+    )
+    result_set.rows.push row
 
     spanner_mock_server.put_statement_result sql, StatementResult.new(result_set)
   end
@@ -727,6 +745,136 @@ def self.register_table_with_sequence_primary_and_parent_key_columns_result span
     register_key_columns_result spanner_mock_server, sql
   end
 
+  def self.register_users_columns_result spanner_mock_server
+    register_commit_timestamps_result spanner_mock_server, "users"
+
+    sql = table_columns_sql "users"
+
+    column_name = Field.new name: "COLUMN_NAME", type: Type.new(code: TypeCode::STRING)
+    spanner_type = Field.new name: "SPANNER_TYPE", type: Type.new(code: TypeCode::STRING)
+    is_nullable = Field.new name: "IS_NULLABLE", type: Type.new(code: TypeCode::STRING)
+    generation_expression = Field.new name: "GENERATION_EXPRESSION", type: Type.new(code: TypeCode::STRING)
+    column_default = Field.new name: "COLUMN_DEFAULT", type: Type.new(code: TypeCode::STRING)
+    ordinal_position = Field.new name: "ORDINAL_POSITION", type: Type.new(code: TypeCode::INT64)
+
+    metadata = ResultSetMetadata.new row_type: StructType.new
+    metadata.row_type.fields.push column_name, spanner_type, is_nullable, generation_expression, column_default, ordinal_position
+    result_set = ResultSet.new metadata: metadata
+
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: "id"),
+      Value.new(string_value: "BYTES(16)"),
+      Value.new(string_value: "NO"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(string_value: "1")
+    )
+    result_set.rows.push row
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: "email"),
+      Value.new(string_value: "STRING(MAX)"),
+      Value.new(string_value: "NO"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(string_value: "2")
+    )
+    result_set.rows.push row
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: "full_name"),
+      Value.new(string_value: "STRING(MAX)"),
+      Value.new(string_value: "NO"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(string_value: "3")
+    )
+    result_set.rows.push row
+
+    spanner_mock_server.put_statement_result sql, StatementResult.new(result_set)
+  end
+
+  def self.register_users_primary_key_columns_result spanner_mock_server
+    sql = primary_key_columns_sql "users", parent_keys: false
+    register_key_columns_result spanner_mock_server, sql
+  end
+
+  def self.register_users_primary_and_parent_key_columns_result spanner_mock_server
+    sql = primary_key_columns_sql "users", parent_keys: true
+    register_key_columns_result spanner_mock_server, sql
+  end
+
+  def self.register_binary_projects_columns_result spanner_mock_server
+    register_commit_timestamps_result spanner_mock_server, "binary_projects"
+
+    sql = table_columns_sql "binary_projects"
+
+    column_name = Field.new name: "COLUMN_NAME", type: Type.new(code: TypeCode::STRING)
+    spanner_type = Field.new name: "SPANNER_TYPE", type: Type.new(code: TypeCode::STRING)
+    is_nullable = Field.new name: "IS_NULLABLE", type: Type.new(code: TypeCode::STRING)
+    generation_expression = Field.new name: "GENERATION_EXPRESSION", type: Type.new(code: TypeCode::STRING)
+    column_default = Field.new name: "COLUMN_DEFAULT", type: Type.new(code: TypeCode::STRING)
+    ordinal_position = Field.new name: "ORDINAL_POSITION", type: Type.new(code: TypeCode::INT64)
+
+    metadata = ResultSetMetadata.new row_type: StructType.new
+    metadata.row_type.fields.push column_name, spanner_type, is_nullable, generation_expression, column_default, ordinal_position
+    result_set = ResultSet.new metadata: metadata
+
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: "id"),
+      Value.new(string_value: "BYTES(16)"),
+      Value.new(string_value: "NO"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(string_value: "1")
+    )
+    result_set.rows.push row
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: "name"),
+      Value.new(string_value: "STRING(MAX)"),
+      Value.new(string_value: "NO"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(string_value: "2")
+    )
+    result_set.rows.push row
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: "description"),
+      Value.new(string_value: "STRING(MAX)"),
+      Value.new(string_value: "NO"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(string_value: "3")
+    )
+    result_set.rows.push row
+    row = ListValue.new
+    row.values.push(
+      Value.new(string_value: "owner_id"),
+      Value.new(string_value: "BYTES(16)"),
+      Value.new(string_value: "NO"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(null_value: "NULL_VALUE"),
+      Value.new(string_value: "4")
+    )
+    result_set.rows.push row
+
+    spanner_mock_server.put_statement_result sql, StatementResult.new(result_set)
+  end
+
+  def self.register_binary_projects_primary_key_columns_result spanner_mock_server
+    sql = primary_key_columns_sql "binary_projects", parent_keys: false
+    register_key_columns_result spanner_mock_server, sql
+  end
+
+  def self.register_binary_projects_primary_and_parent_key_columns_result spanner_mock_server
+    sql = primary_key_columns_sql "binary_projects", parent_keys: true
+    register_key_columns_result spanner_mock_server, sql
+  end
+
   def self.register_empty_select_indexes_result spanner_mock_server, sql
     col_index_name = Field.new name: "INDEX_NAME", type: Type.new(code: TypeCode::STRING)
     col_index_type = Field.new name: "INDEX_TYPE", type: Type.new(code: TypeCode::STRING)
diff --git a/test/activerecord_spanner_mock_server/models/binary_project.rb b/test/activerecord_spanner_mock_server/models/binary_project.rb
new file mode 100644
index 00000000..8e817b37
--- /dev/null
+++ b/test/activerecord_spanner_mock_server/models/binary_project.rb
@@ -0,0 +1,18 @@
+# Copyright 2025 Google LLC
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+
+# frozen_string_literal: true
+
+class BinaryProject < ActiveRecord::Base
+  belongs_to :owner, class_name: 'User'
+
+  before_create :set_uuid
+  private
+
+  def set_uuid
+    self.id ||= StringIO.new(SecureRandom.random_bytes(16))
+  end
+end
diff --git a/test/activerecord_spanner_mock_server/models/user.rb b/test/activerecord_spanner_mock_server/models/user.rb
new file mode 100644
index 00000000..f5fbd330
--- /dev/null
+++ b/test/activerecord_spanner_mock_server/models/user.rb
@@ -0,0 +1,18 @@
+# Copyright 2025 Google LLC
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+
+# frozen_string_literal: true
+
+class User < ActiveRecord::Base
+  has_many :binary_projects, foreign_key: :owner_id
+
+  before_create :set_uuid
+  private
+
+  def set_uuid
+    self.id ||= StringIO.new(SecureRandom.random_bytes(16))
+  end
+end
diff --git a/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb b/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb
index a832e585..2c03f099 100644
--- a/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb
+++ b/test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb
@@ -1259,6 +1259,86 @@ def test_upsert_all_dml
       assert_equal 1, execute_requests.length
     end
 
+    def test_binary_id
+      user = User.create!(
+        email: "test@example.com",
+        full_name: "Test User"
+      )
+      # Verify that an ID was generated for the User.
+      assert user.id
+      assert user.id.is_a?(StringIO)
+
+      commit_requests = @mock.requests.select { |req| req.is_a?(CommitRequest) }
+      assert_equal 1, commit_requests.length
+      assert_equal 1, commit_requests[0].mutations.length
+      mutation = commit_requests[0].mutations[0]
+      assert_equal :insert, mutation.operation
+      assert_equal "users", mutation.insert.table
+
+      assert_equal 1, mutation.insert.values.length
+      assert_equal 3, mutation.insert.values[0].length
+      assert_equal to_base64(user.id), mutation.insert.values[0][0]
+      assert_equal "test@example.com", mutation.insert.values[0][1]
+      assert_equal "Test User", mutation.insert.values[0][2]
+    end
+
+    def test_binary_id_association
+      user = User.create!(
+        email: "test@example.com",
+        full_name: "Test User"
+      )
+      project1 = BinaryProject.create!(
+        name: "Test Project 1",
+        description: "Test Description 1",
+        owner: user
+      )
+      project2 = BinaryProject.create!(
+        name: "Test Project 2",
+        description: "Test Description 2",
+        owner: user
+      )
+      # Verify that an ID was generated for the records.
+      assert user.id
+      assert project1.id
+      assert project2.id
+
+      commit_requests = @mock.requests.select { |req| req.is_a?(CommitRequest) }
+      assert_equal 3, commit_requests.length
+      assert_equal 1, commit_requests[1].mutations.length
+      mutation = commit_requests[1].mutations[0]
+      assert_equal :insert, mutation.operation
+      assert_equal "binary_projects", mutation.insert.table
+
+      assert_equal 1, mutation.insert.values.length
+      assert_equal 4, mutation.insert.values[0].length
+      assert_equal to_base64(project1.id), mutation.insert.values[0][0]
+      assert_equal "Test Project 1", mutation.insert.values[0][1]
+      assert_equal "Test Description 1", mutation.insert.values[0][2]
+      assert_equal to_base64(user.id), mutation.insert.values[0][3]
+    end
+
+    def test_skip_binary_deserialization
+      ENV["SPANNER_BYTES_DESERIALIZE_DISABLED"] = "true"
+      begin
+        user = User.create!(
+          email: "test@example.com",
+          full_name: "Test User"
+        )
+        # Verify that the ID is returned as a Base64 string.
+        assert user.id
+        assert user.id.is_a?(String)
+        assert_equal user.id, Base64.strict_encode64(Base64.strict_decode64(user.id))
+      ensure
+        ENV.delete("SPANNER_BYTES_DESERIALIZE_DISABLED")
+      end
+    end
+
+    def to_base64 buffer
+      buffer.rewind
+      value = buffer.read
+      Base64.strict_encode64 value.force_encoding("ASCII-8BIT")
+    end
+
     private
 
     def verify_insert_upsert_all operation