ruby 48 lines · 8 steps

How a chainable SQL query builder works

A Ruby class that accumulates query parts through chained calls and assembles them into a SQL string on demand.

Explained by highlit
Intermediate fluent-interface method-chaining builder-pattern sql-generation state-accumulation
1class QueryBuilder
2 def initialize(table)
3 @table = table
4 @wheres = []
5 @orders = []
6 @limit = nil
7 @offset = nil
8 @selects = ['*']
9 end
10 
11 def select(*columns)
12 @selects = columns.flatten.map(&:to_s) unless columns.empty?
13 self
14 end
15 
16 def where(condition, *bindings)
17 @wheres << { sql: condition, bindings: bindings }
18 self
19 end
20 
21 def order(column, direction = :asc)
22 @orders << "#{column} #{direction.to_s.upcase}"
23 self
24 end
25 
26 def limit(count)
27 @limit = count.to_i
28 self
29 end
30 
31 def offset(count)
32 @offset = count.to_i
33 self
34 end
35 
36 def to_sql
37 sql = "SELECT #{@selects.join(', ')} FROM #{@table}"
38 sql << " WHERE #{@wheres.map { |w| w[:sql] }.join(' AND ')}" unless @wheres.empty?
39 sql << " ORDER BY #{@orders.join(', ')}" unless @orders.empty?
40 sql << " LIMIT #{@limit}" if @limit
41 sql << " OFFSET #{@offset}" if @offset
42 sql
43 end
44 
45 def bindings
46 @wheres.flat_map { |w| w[:bindings] }
47 end
48end
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Returning self from each mutator method is what enables a fluent, chainable API.
  2. 2Separating accumulation from rendering lets you build state incrementally and generate output only when needed.
  3. 3Keeping bindings apart from the SQL string preserves safe parameterized queries.

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