Add database_exists? method to connection adapters

When SQLite connects it will silently create a database if the database does not
exist. This behaviour causes different issues because of inconsistent behaviour
between adapters: #36383, #32914. This commit adds a `database_exists?` method
as a way to check the database without creating it. This is a stepping stone to
fully resolving the above issues.

8 files changed, 71 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 8d4b01e995..0eea807c80 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Add database_exists? method to connection adapters to check if a database exists.
+
+      *Guilherme Mansur*
+
 *   PostgreSQL: Fix GROUP BY with ORDER BY virtual count attribute.
 
     Fixes #36022.
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index c0ead17b3b..f9edb12759 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -264,6 +264,11 @@ module ActiveRecord
         self.class::ADAPTER_NAME
       end
 
+      # Does the database for this adapter exist?
+      def self.database_exists?(config)
+        raise NotImplementedError
+      end
+
       # Does this adapter support DDL rollbacks in transactions? That is, would
       # CREATE TABLE or ALTER TABLE get rolled back by a transaction?
       def supports_ddl_transactions?
diff --git a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
index 53510c62c2..d9eab9582e 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
@@ -42,6 +42,12 @@ module ActiveRecord
         configure_connection
       end
 
+      def self.database_exists?(config)
+        !!ActiveRecord::Base.mysql2_connection(config)
+      rescue ActiveRecord::NoDatabaseError
+        false
+      end
+
       def supports_json?
         !mariadb? && database_version >= "5.7.8"
       end
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 6b18a12bce..82de52de50 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -259,6 +259,12 @@ module ActiveRecord
         @use_insert_returning = @config.key?(:insert_returning) ? self.class.type_cast_config_to_boolean(@config[:insert_returning]) : true
       end
 
+      def self.database_exists?(config)
+        !!ActiveRecord::Base.postgresql_connection(config)
+      rescue ActiveRecord::NoDatabaseError
+        false
+      end
+
       # Is this connection alive and ready for queries?
       def active?
         @lock.synchronize do
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index e6302112de..3a1edfc855 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -98,6 +98,16 @@ module ActiveRecord
         configure_connection
       end
 
+      def self.database_exists?(config)
+        config = config.symbolize_keys
+        if config[:database] == ":memory:"
+          return true
+        else
+          database_file = defined?(Rails.root) ? File.expand_path(config[:database], Rails.root) : config[:database]
+          File.exist?(database_file)
+        end
+      end
+
       def supports_ddl_transactions?
         true
       end
diff --git a/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb b/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
index e1f7a0b7c5..df84a40f63 100644
--- a/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
+++ b/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
@@ -20,6 +20,18 @@ class Mysql2AdapterTest < ActiveRecord::Mysql2TestCase
     end
   end
 
+  def test_database_exists_returns_false_if_database_does_not_exist
+    config = ActiveRecord::Base.configurations["arunit"].merge(database: "inexistent_activerecord_unittest")
+    assert_not ActiveRecord::ConnectionAdapters::Mysql2Adapter.database_exists?(config),
+      "expected database to not exist"
+  end
+
+  def test_database_exists_returns_true_when_the_database_exists
+    config = ActiveRecord::Base.configurations["arunit"]
+    assert ActiveRecord::ConnectionAdapters::Mysql2Adapter.database_exists?(config),
+     "expected database #{config[:database]} to exist"
+  end
+
   def test_columns_for_distinct_zero_orders
     assert_equal "posts.id",
       @conn.columns_for_distinct("posts.id", [])
diff --git a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
index 68bc87eaf8..d99593817a 100644
--- a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
@@ -24,6 +24,18 @@ module ActiveRecord
         end
       end
 
+      def test_database_exists_returns_false_when_the_database_does_not_exist
+        config = { database: "non_extant_database", adapter: "postgresql" }
+        assert_not ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.database_exists?(config),
+          "expected database #{config[:database]} to not exist"
+      end
+
+      def test_database_exists_returns_true_when_the_database_exists
+        config = ActiveRecord::Base.configurations["arunit"]
+        assert ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.database_exists?(config),
+          "expected database #{config[:database]} to exist"
+      end
+
       def test_primary_key
         with_example_table do
           assert_equal "id", @connection.primary_key("ex")
diff --git a/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb b/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
index 508f7d8945..b6d72c7bcd 100644
--- a/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
+++ b/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
@@ -30,6 +30,17 @@ module ActiveRecord
         end
       end
 
+      def test_database_exists_returns_false_when_the_database_does_not_exist
+        assert_not SQLite3Adapter.database_exists?(adapter: "sqlite3", database: "non_extant_db"),
+          "expected non_extant_db to not exist"
+      end
+
+      def test_database_exists_returns_true_when_databae_exists
+        config = ActiveRecord::Base.configurations["arunit"]
+        assert SQLite3Adapter.database_exists?(config),
+          "expected #{config[:database]} to exist"
+      end
+
       unless in_memory_db?
         def test_connect_with_url
           original_connection = ActiveRecord::Base.remove_connection
@@ -53,6 +64,11 @@ module ActiveRecord
         end
       end
 
+      def test_database_exists_returns_true_for_an_in_memory_db
+        assert SQLite3Adapter.database_exists?(database: ":memory:"),
+          "Expected in memory database to exist"
+      end
+
       def test_column_types
         owner = Owner.create!(name: "hello".encode("ascii-8bit"))
         owner.reload