ruby 40 lines · 7 steps

Building a lazy query proxy in Ruby

A proxy records chainable query calls and replays them against a relation only when results are actually needed.

Explained by highlit
Advanced metaprogramming method_missing lazy-evaluation immutability proxy-pattern
1class QueryProxy
2 ALLOWED = %i[where limit offset order includes distinct].freeze
3 
4 def initialize(relation, operations = [])
5 @relation = relation
6 @operations = operations
7 end
8 
9 def method_missing(name, *args, &block)
10 if ALLOWED.include?(name)
11 self.class.new(@relation, @operations + [[name, args, block]])
12 elsif @relation.respond_to?(name)
13 resolve.public_send(name, *args, &block)
14 else
15 super
16 end
17 end
18 
19 def respond_to_missing?(name, include_private = false)
20 ALLOWED.include?(name) || @relation.respond_to?(name, include_private) || super
21 end
22 
23 def resolve
24 @operations.reduce(@relation) do |rel, (name, args, block)|
25 rel.public_send(name, *args, &block)
26 end
27 end
28 
29 def to_a
30 resolve.to_a
31 end
32 
33 def each(&block)
34 resolve.each(&block)
35 end
36 
37 def explain
38 @operations.map { |name, args, _| "#{name}(#{args.map(&:inspect).join(', ')})" }.join(" -> ")
39 end
40end
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Deferring execution lets you build up a query as data and run it exactly once, on demand.
  2. 2Overriding method_missing must be paired with respond_to_missing? so the object behaves honestly under respond_to? and duck typing.
  3. 3Returning a fresh instance per call keeps each chain step immutable and side-effect free.

Related explainers

ruby 
require "shellwords"
require "open3"
 
module Backup

Building safe shell commands in Ruby

shell-out subprocess command-injection
Intermediate 7 steps
ruby 
class UserAgentParser
  BROWSERS = [
    [/Edg\/([\d.]+)/, "Edge"],
    [/OPR\/([\d.]+)/, "Opera"],

Parsing user-agent strings in Ruby

regex pattern-matching lookup-tables
Intermediate 8 steps
ruby 
class LogAggregator
  BUCKET_FORMAT = "%Y-%m-%dT%H:%M"
 
  def initialize(entries)

Bucketing log entries by the minute in Ruby

aggregation hashing enumerable
Intermediate 5 steps
ruby 
class WeeklySignupsReport
  DEFAULT_WEEKS = 12
 
  def initialize(weeks: DEFAULT_WEEKS, source: User.all)

Building a weekly signups report in Rails

service object aggregation group by
Intermediate 7 steps
rust 
use chrono::{Duration, NaiveDate};
 
#[derive(Debug)]
pub struct DateRange {

Parsing and iterating date ranges in Rust

error-handling iterators parsing
Intermediate 7 steps
go 
package logging
 
import (
	"context"

Deduplicating log attributes in Go's slog

decorator-pattern structured-logging immutability
Intermediate 8 steps

Share this explainer

Here's the card — post it anywhere.

Building a lazy query proxy in Ruby — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code