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Computed Fields

Use computed fields when you need values derived from other fields—totals, averages, conditional statuses, or any value that depends on generated data.

Computed fields derive values from other fields using expressions, aggregates, and arithmetic.

Arithmetic Expressions​

Use standard arithmetic operators:

schema Invoice {
  quantity: int in 1..10,
  unit_price: decimal in 9.99..99.99,

  subtotal: quantity * unit_price,
  tax: subtotal * 0.2,
  total: subtotal + tax
}

Operators: +, -, *, /, % (modulo)

Aggregate Functions​

Aggregate over nested collections:

schema Invoice {
  line_items: 1..10 of LineItem,

  total: sum(line_items.amount),
  item_count: count(line_items),
  avg_price: avg(line_items.unit_price),
  min_price: min(line_items.unit_price),
  max_price: max(line_items.unit_price)
}

Available aggregates:

FunctionDescription
sum(collection.field)Sum of field values
count(collection)Number of items
avg(collection.field)Average of field values
min(collection.field)Minimum value
max(collection.field)Maximum value
median(collection.field)Median value
first(collection.field)First item's value
last(collection.field)Last item's value
product(collection.field)Product of all values

Ternary Expressions​

Conditional values with ? : syntax:

schema Invoice {
  amount_paid: int in 0..1000,
  total: int in 500..1000,

  status: amount_paid >= total ? "paid" : "pending"
}

Nested Ternaries​

Chain for multiple conditions:

schema Student {
  score: int in 0..100,

  grade: score >= 90 ? "A" :
         score >= 80 ? "B" :
         score >= 70 ? "C" :
         score >= 60 ? "D" : "F"
}

With Logical Operators​

schema Order {
  total: decimal in 10..1000,
  is_member: boolean,
  has_coupon: boolean,

  discount: (total >= 100 and is_member) or has_coupon ? 0.15 : 0
}

Rounding Functions​

Control decimal precision:

schema Invoice {
  subtotal: decimal in 100..1000,

  tax: round(subtotal * 0.2, 2),      // Round to 2 decimal places
  floored: floor(subtotal / 3, 1),    // Floor to 1 decimal place
  ceiled: ceil(subtotal / 7, 0)       // Ceil to integer
}

Math Functions​

schema Stats {
  value: decimal in -100..100,

  absolute: abs(value),
  positive: max(0, value),
  clamped: min(100, max(0, value))
}

String Computations​

See String Functions for string transformations:

schema User {
  first_name: firstName(),
  last_name: lastName(),

  full_name: concat(first_name, " ", last_name),
  username: lowercase(concat(first_name, ".", last_name)),
  initials: concat(substring(first_name, 0, 1), substring(last_name, 0, 1))
}

Field Dependencies​

Computed fields can reference other computed fields:

schema Invoice {
  line_items: 1..10 of LineItem,

  subtotal: sum(line_items.amount),    // First
  tax: round(subtotal * 0.2, 2),       // Uses subtotal
  total: subtotal + tax,                // Uses subtotal and tax

  status: total > 1000 ? "large" : "standard"  // Uses total
}

Vague automatically determines the correct evaluation order.

Practical Example​

schema LineItem {
  product: faker.commerce.productName(),
  quantity: int in 1..10,
  unit_price: decimal(2) in 9.99..199.99,
  amount: round(quantity * unit_price, 2)
}

schema Invoice {
  id: sequence("INV-", 1001),
  customer: any of customers,
  line_items: 1..5 of LineItem,

  // Aggregates
  subtotal: sum(line_items.amount),
  item_count: count(line_items),

  // Calculations
  tax_rate: 0.2,
  tax: round(subtotal * tax_rate, 2),
  total: round(subtotal + tax, 2),

  // Derived status
  amount_paid: int in 0..0,
  status: amount_paid >= total ? "paid" : "pending",

  // Metadata
  is_large_order: total > 500
}