1 files changed, 123 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/connection_adapters/schema_cache.rb b/activerecord/lib/active_record/connection_adapters/schema_cache.rb
new file mode 100644
index 0000000000..c10765f42d
--- /dev/null
+++ b/activerecord/lib/active_record/connection_adapters/schema_cache.rb
@@ -0,0 +1,123 @@
+# frozen_string_literal: true
+
+module ActiveRecord
+  module ConnectionAdapters
+    class SchemaCache
+      attr_reader :version
+      attr_accessor :connection
+
+      def initialize(conn)
+        @connection = conn
+
+        @columns      = {}
+        @columns_hash = {}
+        @primary_keys = {}
+        @data_sources = {}
+      end
+
+      def initialize_dup(other)
+        super
+        @columns      = @columns.dup
+        @columns_hash = @columns_hash.dup
+        @primary_keys = @primary_keys.dup
+        @data_sources = @data_sources.dup
+      end
+
+      def encode_with(coder)
+        coder["columns"] = @columns
+        coder["columns_hash"] = @columns_hash
+        coder["primary_keys"] = @primary_keys
+        coder["data_sources"] = @data_sources
+        coder["version"] = connection.migration_context.current_version
+      end
+
+      def init_with(coder)
+        @columns = coder["columns"]
+        @columns_hash = coder["columns_hash"]
+        @primary_keys = coder["primary_keys"]
+        @data_sources = coder["data_sources"]
+        @version = coder["version"]
+      end
+
+      def primary_keys(table_name)
+        @primary_keys[table_name] ||= data_source_exists?(table_name) ? connection.primary_key(table_name) : nil
+      end
+
+      # A cached lookup for table existence.
+      def data_source_exists?(name)
+        prepare_data_sources if @data_sources.empty?
+        return @data_sources[name] if @data_sources.key? name
+
+        @data_sources[name] = connection.data_source_exists?(name)
+      end
+
+      # Add internal cache for table with +table_name+.
+      def add(table_name)
+        if data_source_exists?(table_name)
+          primary_keys(table_name)
+          columns(table_name)
+          columns_hash(table_name)
+        end
+      end
+
+      def data_sources(name)
+        @data_sources[name]
+      end
+
+      # Get the columns for a table
+      def columns(table_name)
+        @columns[table_name] ||= connection.columns(table_name)
+      end
+
+      # Get the columns for a table as a hash, key is the column name
+      # value is the column object.
+      def columns_hash(table_name)
+        @columns_hash[table_name] ||= Hash[columns(table_name).map { |col|
+          [col.name, col]
+        }]
+      end
+
+      # Checks whether the columns hash is already cached for a table.
+      def columns_hash?(table_name)
+        @columns_hash.key?(table_name)
+      end
+
+      # Clears out internal caches
+      def clear!
+        @columns.clear
+        @columns_hash.clear
+        @primary_keys.clear
+        @data_sources.clear
+        @version = nil
+      end
+
+      def size
+        [@columns, @columns_hash, @primary_keys, @data_sources].map(&:size).inject :+
+      end
+
+      # Clear out internal caches for the data source +name+.
+      def clear_data_source_cache!(name)
+        @columns.delete name
+        @columns_hash.delete name
+        @primary_keys.delete name
+        @data_sources.delete name
+      end
+
+      def marshal_dump
+        # if we get current version during initialization, it happens stack over flow.
+        @version = connection.migration_context.current_version
+        [@version, @columns, @columns_hash, @primary_keys, @data_sources]
+      end
+
+      def marshal_load(array)
+        @version, @columns, @columns_hash, @primary_keys, @data_sources = array
+      end
+
+      private
+
+        def prepare_data_sources
+          connection.data_sources.each { |source| @data_sources[source] = true }
+        end
+    end
+  end
+end