Add a basic lookahead object

lib/graphql/execution.rb

@@ -4,5 +4,6 @@
 require "graphql/execution/flatten"
 require "graphql/execution/instrumentation"
 require "graphql/execution/lazy"
+require "graphql/execution/lookahead"
 require "graphql/execution/multiplex"
 require "graphql/execution/typecast"

lib/graphql/execution/lookahead.rb

@@ -0,0 +1,277 @@
+# frozen_string_literal: true
+module GraphQL
+  module Execution
+    # Lookahead creates a uniform interface to inspect the forthcoming selections.
+    #
+    # It assumes that the AST it's working with is valid. (So, it's safe to use
+    # during execution, but if you're using it directly, be sure to validate first.)
+    #
+    # A field may get access to its lookahead by adding `extras: [:lookahead]`
+    # to its configuration.
+    #
+    # __NOTE__: Lookahead for typed fragments (eg `node { ... on Thing { ... } }`)
+    # hasn't been implemented yet. It's possible, I just didn't need it yet.
+    # Feel free to open a PR or an issue if you want to add it.
+    #
+    # @example looking ahead in a field
+    #   field :articles, [Types::Article], null: false,
+    #     extras: [:lookahead]
+    #
+    #   # For example, imagine a faster database call
+    #   # may be issued when only some fields are requested.
+    #   #
+    #   # Imagine that _full_ fetch must be made to satisfy `fullContent`,
+    #   # we can look ahead to see if we need that field. If we do,
+    #   # we make the expensive database call instead of the cheap one.
+    #   def articles(lookahead:)
+    #     if lookahead.selects?(:full_content)
+    #       fetch_full_articles(object)
+    #     else
+    #       fetch_preview_articles(object)
+    #     end
+    #   end
+    class Lookahead
+      # @param query [GraphQL::Query]
+      # @param ast_nodes [Array<GraphQL::Language::Nodes::Field>, Array<GraphQL::Language::Nodes::OperationDefinition>]
+      # @param field [GraphQL::Schema::Field] if `ast_nodes` are fields, this is the field definition matching those nodes
+      # @param root_type [Class] if `ast_nodes` are operation definition, this is the root type for that operation
+      def initialize(query:, ast_nodes:, field: nil, root_type: nil)
+        @ast_nodes = ast_nodes
+        @field = field
+        @root_type = root_type
+        @query = query
+      end
+
+      # True if this node has a selection on `field_name`.
+      # If `field_name` is a String, it is treated as a GraphQL-style (camelized)
+      # field name and used verbatim. If `field_name` is a Symbol, it is
+      # treated as a Ruby-style (underscored) name and camelized before comparing.
+      #
+      # If `arguments:` is provided, each provided key/value will be matched
+      # against the arguments in the next selection. This method will return false
+      # if any of the given `arguments:` are not present and matching in the next selection.
+      # (But, the next selection may contain _more_ than the given arguments.)
+      # @param field_name [String, Symbol]
+      # @param arguments [Hash] Arguments which must match in the selection
+      # @return [Boolean]
+      def selects?(field_name, arguments: nil)
+        selection(field_name, arguments: arguments).selected?
+      end
+
+      # @return [Boolean] True if this lookahead represents a field that was requested
+      def selected?
+        true
+      end
+
+      # Like {#selects?}, but can be used for chaining.
+      # It returns a null object (check with {#selected?})
+      # @return [GraphQL::Execution::Lookahead]
+      def selection(field_name, arguments: nil)
+        next_field_name = normalize_name(field_name)
+
+        next_field_owner = if @field
+          @field.type.unwrap
+        else
+          @root_type
+        end
+
+        next_field_defn = FieldHelpers.get_field(@query.schema, next_field_owner, next_field_name)
+        if next_field_defn
+          next_nodes = []
+          @ast_nodes.each do |ast_node|
+            ast_node.selections.each do |selection|
+              find_selected_nodes(selection, next_field_name, next_field_defn, arguments: arguments, matches: next_nodes)
+            end
+          end
+
+          if next_nodes.any?
+            Lookahead.new(query: @query, ast_nodes: next_nodes, field: next_field_defn)
+          else
+            NULL_LOOKAHEAD
+          end
+        else
+          NULL_LOOKAHEAD
+        end
+      end
+
+      # This is returned for {Lookahead#selection} when a non-existent field is passed
+      class NullLookahead < Lookahead
+        # No inputs required here.
+        def initialize
+        end
+
+        def selected?
+          false
+        end
+
+        def selects?(*)
+          false
+        end
+
+        def selection(*)
+          NULL_LOOKAHEAD
+        end
+      end
+
+      # A singleton, so that misses don't come with overhead.
+      NULL_LOOKAHEAD = NullLookahead.new
+
+      private
+
+      # If it's a symbol, stringify and camelize it
+      def normalize_name(name)
+        if name.is_a?(Symbol)
+          Schema::Member::BuildType.camelize(name.to_s)
+        else
+          name
+        end
+      end
+
+      def normalize_keyword(keyword)
+        if keyword.is_a?(String)
+          Schema::Member::BuildType.underscore(keyword).to_sym
+        else
+          keyword
+        end
+      end
+
+      # If a selection on `node` matches `field_name` (which is backed by `field_defn`)
+      # and matches the `arguments:` constraints, then add that node to `matches`
+      def find_selected_nodes(node, field_name, field_defn, arguments:, matches:)
+        case node
+        when GraphQL::Language::Nodes::Field
+          if node.name == field_name
+            if arguments.nil? || arguments.none?
+              # No constraint applied
+              matches << node
+            else
+              query_kwargs = ArgumentHelpers.arguments(@query, nil, field_defn, node)
+              passes_args = arguments.all? do |arg_name, arg_value|
+                arg_name = normalize_keyword(arg_name)
+                # Make sure the constraint is present with a matching value
+                query_kwargs.key?(arg_name) && query_kwargs[arg_name] == arg_value
+              end
+              if passes_args
+                matches << node
+              end
+            end
+          end
+        when GraphQL::Language::Nodes::InlineFragment
+          node.selections.find { |s| find_selected_nodes(s, field_name, field_defn, arguments: arguments, matches: matches) }
+        when GraphQL::Language::Nodes::FragmentSpread
+          frag_defn = @query.fragments[node.name]
+          frag_defn.selections.find { |s| find_selected_nodes(s, field_name, field_defn, arguments: arguments, matches: matches) }
+        else
+          raise "Unexpected selection comparison on #{node.class.name} (#{node})"
+        end
+      end
+
+      # TODO Dedup with interpreter
+      module ArgumentHelpers
+        module_function
+
+        def arguments(query, graphql_object, arg_owner, ast_node)
+          kwarg_arguments = {}
+          arg_defns = arg_owner.arguments
+          ast_node.arguments.each do |arg|
+            arg_defn = arg_defns[arg.name] || raise("Invariant: missing argument definition for #{arg.name.inspect} in #{arg_defns.keys} from #{arg_owner}")
+            # Need to distinguish between client-provided `nil`
+            # and nothing-at-all
+            is_present, value = arg_to_value(query, graphql_object, arg_defn.type, arg.value)
+            if is_present
+              # This doesn't apply to directives, which are legacy
+              # Can remove this when Skip and Include use classes or something.
+              if graphql_object
+                value = arg_defn.prepare_value(graphql_object, value)
+              end
+              kwarg_arguments[arg_defn.keyword] = value
+            end
+          end
+          arg_defns.each do |name, arg_defn|
+            if arg_defn.default_value? && !kwarg_arguments.key?(arg_defn.keyword)
+              kwarg_arguments[arg_defn.keyword] = arg_defn.default_value
+            end
+          end
+          kwarg_arguments
+        end
+
+        # Get a Ruby-ready value from a client query.
+        # @param graphql_object [Object] The owner of the field whose argument this is
+        # @param arg_type [Class, GraphQL::Schema::NonNull, GraphQL::Schema::List]
+        # @param ast_value [GraphQL::Language::Nodes::VariableIdentifier, String, Integer, Float, Boolean]
+        # @return [Array(is_present, value)]
+        def arg_to_value(query, graphql_object, arg_type, ast_value)
+          if ast_value.is_a?(GraphQL::Language::Nodes::VariableIdentifier)
+            # If it's not here, it will get added later
+            if query.variables.key?(ast_value.name)
+              return true, query.variables[ast_value.name]
+            else
+              return false, nil
+            end
+          elsif ast_value.is_a?(GraphQL::Language::Nodes::NullValue)
+            return true, nil
+          elsif arg_type.is_a?(GraphQL::Schema::NonNull)
+            arg_to_value(query, graphql_object, arg_type.of_type, ast_value)
+          elsif arg_type.is_a?(GraphQL::Schema::List)
+            # Treat a single value like a list
+            arg_value = Array(ast_value)
+            list = []
+            arg_value.map do |inner_v|
+              _present, value = arg_to_value(query, graphql_object, arg_type.of_type, inner_v)
+              list << value
+            end
+            return true, list
+          elsif arg_type.is_a?(Class) && arg_type < GraphQL::Schema::InputObject
+            # For these, `prepare` is applied during `#initialize`.
+            # Pass `nil` so it will be skipped in `#arguments`.
+            # What a mess.
+            args = arguments(query, nil, arg_type, ast_value)
+            # We're not tracking defaults_used, but for our purposes
+            # we compare the value to the default value.
+            return true, arg_type.new(ruby_kwargs: args, context: query.context, defaults_used: nil)
+          else
+            flat_value = flatten_ast_value(query, ast_value)
+            return true, arg_type.coerce_input(flat_value, query.context)
+          end
+        end
+
+        def flatten_ast_value(query, v)
+          case v
+          when GraphQL::Language::Nodes::Enum
+            v.name
+          when GraphQL::Language::Nodes::InputObject
+            h = {}
+            v.arguments.each do |arg|
+              h[arg.name] = flatten_ast_value(query, arg.value)
+            end
+            h
+          when Array
+            v.map { |v2| flatten_ast_value(query, v2) }
+          when GraphQL::Language::Nodes::VariableIdentifier
+            flatten_ast_value(query.variables[v.name])
+          else
+            v
+          end
+        end
+      end
+
+      # TODO dedup with interpreter
+      module FieldHelpers
+        module_function
+
+        def get_field(schema, owner_type, field_name )
+          field_defn = owner_type.get_field(field_name)
+          field_defn ||= if owner_type == schema.query.metadata[:type_class] && (entry_point_field = schema.introspection_system.entry_point(name: field_name))
+            entry_point_field.metadata[:type_class]
+          elsif (dynamic_field = schema.introspection_system.dynamic_field(name: field_name))
+            dynamic_field.metadata[:type_class]
+          else
+            raise "Invariant: no field for #{owner_type}.#{field_name}"
+          end
+
+          field_defn
+        end
+      end
+    end
+  end
+end

lib/graphql/query/context.rb

@@ -74,6 +74,12 @@ def backtrace
         def execution_errors
           @execution_errors ||= ExecutionErrors.new(self)
         end
+
+        def lookahead
+          ast_nodes = irep_node.ast_nodes
+          field = irep_node.definition.metadata[:type_class] || raise("Lookahead is only compatible with class-based schemas")
+          Execution::Lookahead.new(query: query, ast_nodes: ast_nodes, field: field)
+        end
       end
 
       class ExecutionErrors

lib/graphql/schema/argument.rb

@@ -51,6 +51,14 @@ def initialize(arg_name = nil, type_expr = nil, desc = nil, required:, type: nil
         end
       end
 
+      # @return [Object] the value used when the client doesn't provide a value for this argument
+      attr_reader :default_value
+
+      # @return [Boolean] True if this argument has a default value
+      def default_value?
+        @default_value != NO_DEFAULT
+      end
+
       attr_writer :description
 
       # @return [String] Documentation for this argument

lib/graphql/schema/member/has_fields.rb

@@ -66,6 +66,15 @@ def fields
           all_fields
         end
 
+        def get_field(field_name)
+          for ancestor in ancestors
+            if ancestor.respond_to?(:own_fields) && f = ancestor.own_fields[field_name]
+              return f
+            end
+          end
+          nil
+        end
+
         # Register this field with the class, overriding a previous one if needed.
         # Also, add a parent method for resolving this field.
         # @param field_defn [GraphQL::Schema::Field]