yByX / y_by_x

Ruby

rel.y_by_x(:value_attr, :key_attr)

TypeScript

rel.yByX('valueAttr', 'keyAttr')

Problem

Create a lookup object/hash from a relation where one attribute becomes the key and another becomes the value.

Example: I want to create a mapping from product IDs to product names for quick lookups.

Description

This operator creates a Hash/object where keys come from one attribute and values from another. This is useful for creating lookup tables or dictionaries from relational data.

Parameters:

• First argument (y) - The attribute to use as values
• Second argument (x) - The attribute to use as keys

The result is a plain Hash/object (not a relation).

Requirements

• Both specified attributes must exist in the relation
• The key attribute values should be unique (later values overwrite earlier ones for duplicate keys)

Examples

Basic lookup table

Ruby

suppliers = Bmg::Relation.new([
  { sid: "S1", name: "Smith", city: "London" },
  { sid: "S2", name: "Jones", city: "Paris" },
  { sid: "S3", name: "Blake", city: "Paris" },
])

# Create a sid -> name mapping
name_by_id = suppliers.y_by_x(:name, :sid)

# => { "S1" => "Smith", "S2" => "Jones", "S3" => "Blake" }

# Use for quick lookups
name_by_id["S1"]  # => "Smith"

TypeScript

const suppliers = Bmg([
  { sid: "S1", name: "Smith", city: "London" },
  { sid: "S2", name: "Jones", city: "Paris" },
  { sid: "S3", name: "Blake", city: "Paris" },
])

// Create a sid -> name mapping
const nameById = suppliers.yByX('name', 'sid')

// => { S1: "Smith", S2: "Jones", S3: "Blake" }

// Use for quick lookups
console.log(nameById['S1'])  // => "Smith"

Creating a price lookup

Ruby

products = Bmg::Relation.new([
  { sku: "WIDGET-01", name: "Widget", price: 9.99 },
  { sku: "GADGET-02", name: "Gadget", price: 19.99 },
])

price_by_sku = products.y_by_x(:price, :sku)

# => { "WIDGET-01" => 9.99, "GADGET-02" => 19.99 }

TypeScript

const products = Bmg([
  { sku: "WIDGET-01", name: "Widget", price: 9.99 },
  { sku: "GADGET-02", name: "Gadget", price: 19.99 },
])

const priceBySku = products.yByX('price', 'sku')

// => { "WIDGET-01": 9.99, "GADGET-02": 19.99 }

Note on duplicates

If multiple tuples have the same key value, later values will overwrite earlier ones:

Ruby

data = Bmg::Relation.new([
  { key: "A", value: 1 },
  { key: "A", value: 2 },  # Overwrites previous
])

data.y_by_x(:value, :key)
# => { "A" => 2 }  # Only one value is kept

TypeScript

const data = Bmg([
  { key: "A", value: 1 },
  { key: "A", value: 2 },  // Overwrites previous
])

data.yByX('value', 'key')
// => { A: 2 }  // Only the last value is kept

To handle duplicates, consider using summarize with collect first.