class GraphQL::StaticValidation::FieldsHaveAppropriateSelections

def validate(context)
  context.visitor[GraphQL::Language::Nodes::Field] << ->(node, parent)  {
    field_defn = context.field_definition
    validate_field_selections(node, field_defn.type.unwrap, context)
  }
  context.visitor[GraphQL::Language::Nodes::OperationDefinition] << ->(node, parent)  {
    validate_field_selections(node, context.type_definition, context)
  }
end

def validate_field_selections(ast_node, resolved_type, context)
  msg = if resolved_type.nil?
    nil
  elsif resolved_type.kind.scalar? && ast_node.selections.any?
    if ast_node.selections.first.is_a?(GraphQL::Language::Nodes::InlineFragment)
      "Selections can't be made on scalars (%{node_name} returns #{resolved_type.name} but has inline fragments [#{ast_node.selections.map(&:type).map(&:name).join(", ")}])"
    else
      "Selections can't be made on scalars (%{node_name} returns #{resolved_type.name} but has selections [#{ast_node.selections.map(&:name).join(", ")}])"
    end
  elsif resolved_type.kind.fields? && ast_node.selections.none?
    "Field must have selections (%{node_name} returns #{resolved_type.name} but has no selections. Did you mean '#{ast_node.name} { ... }'?)"
  else
    nil
  end
  if msg
    node_name = case ast_node
    when GraphQL::Language::Nodes::Field
      "field '#{ast_node.name}'"
    when GraphQL::Language::Nodes::OperationDefinition
      if ast_node.name.nil?
        "anonymous query"
      else
        "#{ast_node.operation_type} '#{ast_node.name}'"
      end
    else
      raise("Unexpected node #{ast_node}")
    end
    context.errors << message(msg % { node_name: node_name }, ast_node, context: context)
    GraphQL::Language::Visitor::SKIP
  end
end