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.

4 files changed, 27 insertions, 0 deletions

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