constants

Ruby

rel.constants(status: "active")
rel.constants(version: 1, source: "api")

TypeScript

rel.constants({ status: "active" })
rel.constants({ version: 1, source: "api" })

Problem

Add fixed-value attributes to every tuple in a relation.

Example: I want to tag all records from a particular data source with a constant identifier.

Description

The constants operator extends each tuple with new attributes that have the same value for every tuple. This is a specialized form of extend optimized for static values rather than computed ones.

Unlike extend, which can compute values from other attributes, constants simply merges the provided values into each tuple. This makes it more efficient when you just need to add fixed metadata or tags.

Requirements

No special requirements. Works on any relation.

Examples

Adding a single constant

Ruby

data = Bmg::Relation.new([
  { id: 1, name: "Alice" },
  { id: 2, name: "Bob" }
])

data.constants(active: true).to_a

=>
[{:id=>1, :name=>"Alice", :active=>true},
 {:id=>2, :name=>"Bob", :active=>true}]

TypeScript

const data = Bmg([
  { id: 1, name: "Alice" },
  { id: 2, name: "Bob" }
])

data.constants({ active: true }).toArray()

// =>
// [{ id: 1, name: "Alice", active: true },
//  { id: 2, name: "Bob", active: true }]

Multiple constants

Ruby

records = Bmg::Relation.new([
  { id: 1, value: 100 },
  { id: 2, value: 200 }
])

records.constants(
  source: "import",
  version: 2,
  processed_at: "2024-01-15"
).to_a

=>
[{:id=>1, :value=>100, :source=>"import", :version=>2, :processed_at=>"2024-01-15"},
 {:id=>2, :value=>200, :source=>"import", :version=>2, :processed_at=>"2024-01-15"}]

TypeScript

const records = Bmg([
  { id: 1, value: 100 },
  { id: 2, value: 200 }
])

records.constants({
  source: "import",
  version: 2,
  processed_at: "2024-01-15"
}).toArray()

// =>
// [{ id: 1, value: 100, source: "import", version: 2, processed_at: "2024-01-15" },
//  { id: 2, value: 200, source: "import", version: 2, processed_at: "2024-01-15" }]

Comparison with extend

Operator	Use case	Values
constants	Add fixed metadata	Same value for all tuples
extend	Compute new attributes	Different value per tuple

Ruby

# constants: same value everywhere
rel.constants(version: 1)

# extend: computed per tuple
rel.extend(doubled: ->(t) { t[:value] * 2 })

TypeScript

// constants: same value everywhere
rel.constants({ version: 1 })

// extend: computed per tuple
rel.extend({ doubled: t => t.value * 2 })

Use constants when adding static values - it’s simpler and may be more efficient than extend with a constant function.