Filter Expressions

Building thoughtful indexes can make queries simple and performant, though sometimes you need to apply further filters to your query.

FilterExpressions

Below demonstrates how to add a FilterExpression to Dynamo’s DocumentClient directly alongside how you would accomplish the same using the where method.

Example

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa" })
  .where(
    ({ isPregnant, offspring }, { exists, eq }) => `
    ${eq(isPregnant, true)} OR ${exists(offspring)}
  `,
  )
  .go();

Where Clause

The where() method allow you to write a FilterExpression or ConditionExpression without having to worry about the complexities of expression attributes. To accomplish this, ElectroDB injects an object attributes as the first parameter to all Filter Functions, and an object operations, as the second parameter. Pass the properties from the attributes object to the methods found on the operations object, along with inline values to set filters and conditions.

Provided where callbacks must return a string. All method on the operation object all return strings, so you can return the results of the operation method or use template strings compose an expression.

Examples

A single filter expression

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa", enclosure: "5b" })
  .where((attr, op) => op.eq(attr.dangerous, true))
  .go();

A single filter expression with destructuring

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa", enclosure: "5b" })
  .where(({ dangerous }, { eq }) => eq(dangerous, true))
  .go();

Multiple filter expressions

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa", enclosure: "5b" })
  .where(
    (attr, op) => `
    ${op.eq(attr.dangerous, true)} AND ${op.notExists(attr.lastFed)}
  `,
  )
  .go();

Chained usage (implicit AND)

import { animals } from "./entity";

animals.query
  .habitats({ habitat: "Africa", enclosure: "5b" })
  .where((attr, op) => `
    ${op.eq(attr.dangerous, true)} OR ${op.notExists(attr.lastFed)}
  `)
  .where(({ birthday }, { between }) => {
    const today = Date.now();
    const lastMonth = today - 1000 * 60 * 60 * 24 * 30;
    return between(birthday, lastMonth, today);
  })
  .go();

“dynamic” filtering

import { animals } from "./entity";

type GetAnimalOptions = {
  habitat: string;
  keepers: string[];
};

function getAnimals(options: GetAnimalOptions) {
  const { habitat, keepers } = options;
  return animals.query.exhibit({ habitat })
  .where(({ keeper }, { eq }) => {
    return keepers.map((name) => eq(keeper, name)).join(' AND ');
  }).go()
}

const { data, cursor } = await getAnimals({
  habitat: "RainForest",
  keepers: ["Joe Exotic", "Carol Baskin"],
});

Operations

The attributes object contains every Attribute defined in the Entity’s Model. The operations object contains the following methods:

ElectroDB Functions

The filter functions available above all come from the list of functions supported by DynamoDB directly. ElectroDB offers a few functions that offer some additional convenience when creating Filter Expressions.

Note: If ElectroDB ever lags behind in implementing FilterExpression functions, you can use value(), name(), escape(), and/or field() to simply template out the implementation!

Name

The name() function allows you to create a reference to an attribute’s name, which can be useful to create filters referencing an attribute as it is currently stored. The function will add a ExpressionAttributeNames property for this record and return the partial expression for use in your filter.

This example demonstrates how you might find animals that were last fed by someone other than their “keeper”.

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa", enclosure: "5b" })
  .where(
    ({ lastFedBy, keeper }, { name }) => `
    ${name(lastFedBy)} != ${name(keeper)}
  `,
  )
  .go();

Value

The value() function can be paired with name() as convenience when building unique filters. The value() method is passed an attribute (used to enforce type), and a value for that attribute. ElectroDB will add a ExpressionAttributeValues property for this value, and return a reference to that value that can be used in your FilterExpression.ExpressionAttributeValues

This example shows how you can implement the eq operation without using the ElectroDB eq function

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa", enclosure: "5b" })
  .where(
    ({ keeper }, { name, value, eq }) => `
    ${name(keeper)} = ${value(keeper, "Tiger King")}
  `,
  )
  .go();

Escape

The escape() method allows you to provide any primitive to an expression without the need for a specific attribute. In many ways, this function is like a less restrictive version of value(). This can be useful when using static values and/or when creating custom FilterExpressions

This example shows how you might use escape to apply a filter against the size() of an attribute.

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa", enclosure: "5b" })
  .where(
    ({ diet }, { size, escape }) => `
    ${size(diet)} > ${escape(2)}
  `,
  )
  .go();

Field

The field() method allows you to provide and reference a field value that is not present in your model. This method is similar to escape() but is used for field names.

You can use both escape() and field() to template any filter that DynamoDB supports.

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Africa", enclosure: "5b" })
  .where(
    (_, { field, escape }) => `
    contains(${field("gsi1sk")}, ${escape("value")})
  `,
  )
  .go();

Advanced Usage

Where with Complex Attributes

ElectroDB supports using the where() method with DynamoDB’s complex attribute types: map, list, and set. When using the injected attributes object, simply drill into the attribute itself to apply your update directly to the required object.

The following are examples on how to filter on complex attributes:

Filtering on a map attribute

import { animals } from "./entity";

animals.query
  .farm({ habitat: "Africa" })
  .where(({ veterinarian }, { eq }) => eq(veterinarian.name, "Herb Peterson"))
  .go();

Filtering on an element in a list attribute

import { animals } from "./entity";

animals.query
  .exhibit({ habitat: "Tundra" })
  .where(({ offspring }, { eq }) => eq(offspring[0].name, "Blitzen"))
  .go();

Multiple Where Clauses

It is possible to include chain multiple where clauses. The resulting FilterExpressions (or ConditionExpressions) are concatenated with an implicit AND operator.

import { MallStores } from "./entity";

let stores = await MallStores.query
  .leases({ mallId: "EastPointe" })
  .between({ leaseEndDate: "2020-04-01" }, { leaseEndDate: "2020-07-01" })
  .where(
    ({ rent, discount }, { between, eq }) => `
		${between(rent, "2000.00", "5000.00")} AND ${eq(discount, "1000.00")}
	`,
  )
  .where(
    ({ category }, { eq }) => `
		${eq(category, "food/coffee")}
	`,
  )
  .go();