set.map()
Description
The map() method creates a Set by applying the functionBody
to each value in the Set, and returns the new Set. The source
Set isn’t changed.
If map() is the last value in a query, the first page of the new
Set is returned.
Writes are not permitted by functionBody.
Iterator methods
FQL provides several methods for iterating over a Set.
set.forEach(),
set.map(),
set.flatMap() are similar but used
for different purposes:
| Method | Primary use | Notes |
|---|---|---|
Perform in-place writes on Set elements. |
Doesn’t return a value. |
|
Returns a new Set. |
Can’t perform writes. |
|
Similar to |
Can’t perform writes. |
For examples, see:
Examples
Basic example
For all Customer documents, combine the address.city and address.state
properties into a single string:
Customer.all().map(
customer => {
name: customer.name,
city: "#{customer.address.city}, #{customer.address.state}"
}
){
data: [
{
name: "Alice Appleseed",
city: "Washington, DC"
},
{
name: "Bob Brown",
city: "Washington, DC"
},
{
name: "Carol Clark",
city: "Washington, DC"
}
]
}Project Sets using set.map()
You can use set.map() to project a Set, similar to using
Set projection.
For example, the following projection query:
Product.sortedByPriceLowToHigh() {
name,
description,
price
}Is equivalent to the following set.map() query:
Product.sortedByPriceLowToHigh().map(prod => {
name: prod.name,
description: prod.description,
price: prod.price,
})set.forEach() vs. set.map()
You can use both set.forEach() and
set.map() to iterate through a Set.
Use set.forEach() to perform
in-place writes on the calling Set:
// Gets the frozen category.
let frozen = Category.byName("frozen").first()
// Uses `forEach()` to delete each product in
// the frozen category.
Product.byCategory(frozen).forEach(product => {
product.delete()
})nullAlthough it returns null,
set.forEach() still performs the
requested operations.
Unlike set.forEach(),
set.map() can’t perform writes:
// Gets the produce category.
let produce = Category.byName("produce").first()
// Attempts to use `map()` to delete each product in
// the produce category.
Product.byCategory(produce).map(product => {
product.delete()
})invalid_effect: `delete` performs a write, which is not allowed in set functions.
error: `delete` performs a write, which is not allowed in set functions.
at *query*:7:17
|
7 | product.delete()
| ^^
|Instead, you can use set.map() to
output a new Set containing extracted or transformed values:
// Gets the produce category.
let produce = Category.byName("produce").first()
// Uses `map()` to outputs a new Set containing products in
// the produce category. The new Set transforms each product's
// name.
Product.byCategory(produce).map(product => {
let product: Any = product
let category: Any = product.category
{
name: category.name + ": " + product.name,
}
}){
data: [
{
name: "produce: avocados"
},
{
name: "produce: single lime"
},
{
name: "produce: organic limes"
},
{
name: "produce: limes"
},
{
name: "produce: cilantro"
}
]
}set.map() vs. set.flatMap()
set.flatMap() is similar to
set.map(), except
set.flatMap() also flattens the
resulting Set by one level.
In the following example, set.map()
returns a two-dimensional Set:
// Get a Set of all `Category` collection documents.
let categories = Category.all()
// Use `map()` to get a Set of `Product` documents
// for each category.
categories.map(category => {
let category: Any = category
Product.byCategory(category)
})// Two-dimensional Set.
{
data: [
{
data: [
{
id: "111",
coll: Product,
ts: Time("2099-10-02T22:37:39.583Z"),
name: "cups",
description: "Translucent 9 Oz, 100 ct",
price: 698,
stock: 100,
category: Category("123")
},
...
]
},
{
data: [
{
id: "333",
coll: Product,
ts: Time("2099-10-02T22:37:39.583Z"),
name: "pizza",
description: "Frozen Cheese",
price: 499,
stock: 100,
category: Category("456")
}
]
},
{
data: [
{
id: "444",
coll: Product,
ts: Time("2099-10-02T22:37:39.583Z"),
name: "avocados",
description: "Conventional Hass, 4ct bag",
price: 399,
stock: 1000,
category: Category("789")
},
...
]
}
]
}To flatten the result to a one-dimensional array, use
set.flatMap() instead:
// Get a Set of all `Category` collection documents.
let categories = Category.all()
// Use `flatMap()` to get a Set of `Product` documents
// for each category. Then flatten the resulting Set.
categories.flatMap(category => {
let category: Any = category
Product.byCategory(category)
})// One-dimensional Set.
{
data: [
{
id: "111",
coll: Product,
ts: Time("2099-10-02T22:37:39.583Z"),
name: "cups",
description: "Translucent 9 Oz, 100 ct",
price: 698,
stock: 100,
category: Category("123")
},
...
{
id: "333",
coll: Product,
ts: Time("2099-10-02T22:37:39.583Z"),
name: "pizza",
description: "Frozen Cheese",
price: 499,
stock: 100,
category: Category("456")
},
...
{
id: "444",
coll: Product,
ts: Time("2099-10-02T22:37:39.583Z"),
name: "avocados",
description: "Conventional Hass, 4ct bag",
price: 399,
stock: 1000,
category: Category("789")
}
]
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