mongodb/mongoid · error · Mongoid::Errors::InvalidQuery

Expression must be a Hash: #

Error message

Expression must be a Hash: #{Errors::InvalidQuery.truncate_expr(criterion)}

What it means

Raised by Selectable#expr_query when the expression argument is not a Hash. expr_query builds a raw MongoDB query expression and requires field/value pairs; the faulting input is the non-Hash criterion (truncated in the message).

Solutions

1. Pass a Hash such as { age: 50 } to expr_query
2. Build operator conditions as hashes, e.g. { field: { '$gt' => 1 } }

When it happens

Trigger: Thrown at lib/mongoid/criteria/queryable/selectable.rb:813 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of mongodb/mongoid@0da0e23d71 (2026-08-23). Data as JSON: /api/errors/aebd8b3fe9bfcdad. Report an issue: GitHub.

Appendix: source

Thrown at lib/mongoid/criteria/queryable/selectable.rb:813

        #
        # @example Create the selection.
        #   selectable.expr_query(age: 50)
        #
        # @param [ Hash ] criterion The field/value pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        # @api private
        # Operators permitted in a query expression without opt-in.
        # Excludes $where (JS execution) and other operators not needed for
        # ordinary application queries.
        ALLOWED_QUERY_OPERATORS = %w[
          $and $or $nor $not $text $comment $expr $jsonSchema $alwaysFalse $alwaysTrue
        ].freeze

        def expr_query(criterion)
          raise ArgumentError, 'Criterion cannot be nil here' if criterion.nil?
          unless criterion.is_a?(Hash)
            raise Errors::InvalidQuery, "Expression must be a Hash: #{Errors::InvalidQuery.truncate_expr(criterion)}"
          end

          normalized = _mongoid_expand_keys(criterion)
          clone.tap do |query|
            normalized.each do |field, value|
              field_s = field.to_s
              if field_s.start_with?('$')
                unless Mongoid.allow_unsafe_query_operators? || ALLOWED_QUERY_OPERATORS.include?(field_s)
                  raise Errors::InvalidQuery,
                        "Operator '#{field_s}' is not allowed in a query expression. " \
                        'Set Mongoid.allow_unsafe_query_operators = true to permit all operators.'
                end
                query.add_operator_expression(field_s, value)
              else
                query.add_field_expression(field, value)
              end
            end
            query.reset_strategies!

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