transform

Ruby

rel.transform(attr: :upcase)
rel.transform(attr: ->(v) { v * 2 })
rel.transform(attr: Integer)
rel.transform(:downcase)
rel.transform { |v| v.to_s }

TypeScript

rel.transform({ attr: v => v.toUpperCase() })
rel.transform({ attr: v => v * 2 })

Problem

Modify existing attribute values in place using transformations.

Example: I want to normalize all string values to uppercase, or convert all numeric strings to integers.

Description

The transform operator modifies existing attribute values (unlike extend which adds new attributes).

Ruby

It supports several transformation styles:

Per-attribute transformation (Hash): attr: :method (call method), attr: Class (type conversion), attr: ->(v) { ... } (lambda), attr: { old => new } (value mapping), or Class => transformer (by type).

All-attributes transformation: Pass a Symbol (:downcase) or Proc to transform every value.

TypeScript

Transformations are specified as an object where keys are attribute names and values are functions that transform the attribute value.

Requirements

No special requirements. Works on any relation.

Examples

Transform specific attributes

Ruby

data = Bmg::Relation.new([
  { id: 1, name: "alice", email: "ALICE@EXAMPLE.COM" },
  { id: 2, name: "bob", email: "BOB@EXAMPLE.COM" }
])

data.transform(name: :capitalize, email: :downcase).to_a

=>
[{:id=>1, :name=>"Alice", :email=>"alice@example.com"},
 {:id=>2, :name=>"Bob", :email=>"bob@example.com"}]

TypeScript

const data = Bmg([
  { id: 1, name: "alice", email: "ALICE@EXAMPLE.COM" },
  { id: 2, name: "bob", email: "BOB@EXAMPLE.COM" }
])

data.transform({
  name: v => v.charAt(0).toUpperCase() + v.slice(1),
  email: v => v.toLowerCase()
}).toArray()

// =>
// [{ id: 1, name: "Alice", email: "alice@example.com" },
//  { id: 2, name: "Bob", email: "bob@example.com" }]

Lambda transformation

Ruby

orders = Bmg::Relation.new([
  { id: 1, total: 100, discount: 0.1 },
  { id: 2, total: 200, discount: 0.2 }
])

orders.transform(
  total: ->(v) { v.round(2) },
  discount: ->(v) { "#{(v * 100).to_i}%" }
).to_a

=>
[{:id=>1, :total=>100, :discount=>"10%"},
 {:id=>2, :total=>200, :discount=>"20%"}]

TypeScript

const orders = Bmg([
  { id: 1, total: 100.456, discount: 0.1 },
  { id: 2, total: 200.789, discount: 0.2 }
])

orders.transform({
  total: v => Math.round(v * 100) / 100,
  discount: v => `${Math.round(v * 100)}%`
}).toArray()

// =>
// [{ id: 1, total: 100.46, discount: "10%" },
//  { id: 2, total: 200.79, discount: "20%" }]

Comparison with extend

Operator	Purpose	Result
transform	Modify existing values	Same attributes, different values
extend	Add new attributes	Additional attributes

Ruby

# transform: modifies existing attribute
rel.transform(name: :upcase)
# { name: "alice" } => { name: "ALICE" }

# extend: adds new attribute
rel.extend(upper_name: ->(t) { t[:name].upcase })
# { name: "alice" } => { name: "alice", upper_name: "ALICE" }

TypeScript

// transform: modifies existing attribute
rel.transform({ name: v => v.toUpperCase() })
// { name: "alice" } => { name: "ALICE" }

// extend: adds new attribute
rel.extend({ upper_name: t => t.name.toUpperCase() })
// { name: "alice" } => { name: "alice", upper_name: "ALICE" }