Entity Loaders

If you’re developing a new feature or just generally need to get at some of the application data on the frontend, Entity Loaders are going to be your friend. They abstract away calling the API, handling loading and error state, cache previously loaded objects, invalidating the cache (in some cases) and let you easily perform updates, or create new items.

Good uses for Entity Loaders

  • I need to get a specific X (user, database, etc) and display it.
  • I need to get a list of X (databases, questions, etc) and display it.

Currently available entities:

  • Questions, Dashboards, Pulses
  • Collections
  • Databases, Tables, Fields, Segments, Metrics
  • Users, Groups
  • Full current list of entities here:

There are two ways to use loaders, either as React “render prop” components or as React component class decorators (“higher order components”).

Object loading

In this example we’re going to load information about a specific database for a new page.

import React from "react";
import Databases from "metabase/entities/databases";

@Databases.load({ id: 4 })
class MyNewPage extends React.Component {
  render() {
    const { database } = this.props;
    return (

This example uses a class decorator to ask for and then display a database with ID 4. If you instead wanted to use a render prop component your code would look like this.

import React from "react";
import Databases from "metabase/entities/databases";

class MyNewPage extends React.Component {
  render() {
    const { database } = this.props;
    return (
        <Databases.Loader id={4}>
          {({ database }) => <h1>{}</h1>}

Now you most likely don’t just want to display just one static item so for cases where some of the values you might need will be dynamic you can use a function to get at the props and return the value you need. If you’re using the component approach you can just pass props as you would normally for dynamic values.

  id: (state, props) => props.params.databaseId

List loading

Loading a list of items is as easy as applying the loadList decorator:

import React from "react";
import Users from "metabase/entities/users";

class MyList extends React.Component {
  render() {
    const { users } = this.props;
    return <div>{ => u.first_name)}</div>;

Similar to the object loader’s id argument you can also pass a query object (if the API supports it):

  query: (state, props) => ({ archived: props.showArchivedOnly })

Control over loading and error states

By default both EntityObject and EntityList loaders will handle loading state for you by using LoadingAndErrorWrapper under the hood. If for some reason you want to handle loading on your own you can disable this behavior by setting loadingAndErrorWrapper: false.

Wrapped objects

If you pass wrapped: true to a loader then the object or objects will be wrapped with helper classes that let you do things like user.getName(), user.delete(), or user.update({ name: "new name" ). Actions are automatically already bound to dispatch.

This may incur a performance penalty if there are many objects.

Any additional selectors and actions defined in the entities’ objectSelectors or objectActions will appear as the wrapped object’s methods.

Advanced usage

You can also use the Redux actions and selectors directly, for example, dispatch(Users.actions.loadList()) and Users.selectors.getList(state).

Style Guide

Set up Prettier

We use Prettier to format our JavaScript code, and it is enforced by CI. We recommend setting your editor to “format on save”. You can also format code using yarn prettier, and verify it has been formatted correctly using yarn lint-prettier.

We use ESLint to enforce additional rules. It is integrated into the Webpack build, or you can manually run yarn lint-eslint to check.

React and JSX Style Guide

For the most part we follow the Airbnb React/JSX Style Guide. ESLint and Prettier should take care of a majority of the rules in the Airbnb style guide. Exceptions will be noted in this document.

  • Prefer React function components over class components
  • Avoid creating new components within the containers folder, as this approach has been deprecated. Instead, store both connected and view components in the components folder for a more unified and efficient organization. If a connected component grows substantially in size and you need to extract a view component, opt for using the View suffix.
  • For control components, typically we use value and onChange. Controls that have options (e.x. Radio, Select) usually take an options array of objects with name and value properties.
  • Components named like FooModal and FooPopover typically refer to the modal/popover content which should be used inside a Modal/ModalWithTrigger or Popover/PopoverWithTrigger
  • Components named like FooWidget typically include a FooPopover inside a PopoverWithTrigger with some sort of trigger element, often FooName

  • Use arrow function instance properties if you need to bind a method in a class (instead of this.method = this.method.bind(this); in the constructor), but only if the function needs to be bound (e.x. if you’re passing it as a prop to a React component)
class MyComponent extends React.Component {
  constructor(props) {
    // NO:
    this.handleChange = this.handleChange.bind(this);
  // YES:
  handleChange = e => {
    // ...
  // no need to bind:
  componentDidMount() {}
  render() {
    return <input onChange={this.handleChange} />;
  • For styling components we currently use a mix of styled-components and “atomic” / “utility-first” CSS classes.
  • Prefer using grid-styled’s Box and Flex components over raw div.
  • Components should typically pass along their className prop to the root element of the component. It can be merged with additional classes using the cx function from the classnames package.
  • In order to make components more reusable, a component should only apply classes or styles to the root element of the component which affects the layout/styling of it’s own content, but not the layout of itself within it’s parent container. For example, it can include padding or the flex class, but it shouldn’t include margin or flex-full, full, absolute, spread, etc. Those should be passed via className or style props by the consumer of the component, which knows how the component should be positioned within itself.
  • Avoid breaking JSX up into separate method calls within a single component. Prefer inlining JSX so that you can better see what the relation is of the JSX a render method returns to what is in the state or props of a component. By inlining JSX you’ll also get a better sense of what parts should and should not be separate components.

// don't do this
render () {
  return (
      {this.state.thing3Needed && this.renderThing3()}

// do this
render () {
  return (
      <button onClick={this.toggleThing3Needed}>toggle</button>
      <Thing2 randomProp={} />
      {this.state.thing3Needed && <Thing3 randomProp2={} />}

JavaScript Conventions

  • imports should be ordered by type, typically:
    1. external libraries (react is often first, along with things like ttags, underscore, classnames, etc)
    2. Metabase’s top-level React components and containers (metabase/components/*, metabase/containers/*, etc)
    3. Metabase’s React components and containers specific to this part of the application (metabase/*/components/* etc)
    4. Metabase’s libs, entities, services, Redux files, etc
  • Prefer const to let (and never use var). Only use let if you have a specific reason to reassign the identifier (note: this now enforced by ESLint)
  • Prefer arrow functions for inline functions, especially if you need to reference this from the parent scope (there should almost never be a need to do const self = this; etc), but usually even if you don’t (e.x. => x * 2)).
  • Prefer function declarations for top-level functions, including React function components. The exception is for one-liner functions that return a value
// YES:
function MyComponent(props) {
  return <div>...</div>;
// NO:
const MyComponent = props => {
  return <div>...</div>;
// YES:
const double = n => n * 2;
function double(n) {
  return n * 2;
  • Prefer native Array methods over underscore’s. We polyfill all ES6 features. Use Underscore for things that aren’t implemented natively.
  • Prefer async/await over using promise.then(...) etc directly.
  • You may use assignment destructuring or argument destructuring, but avoid deeply nested destructuring, since they can be hard to read and prettier sometimes formats them with extra whitespace.
    • avoid destructuring properties from “entity”-like objects, e.x. don’t do const { display_name } = column;
    • don’t destructure this directly, e.x. const { foo } = this.props; const { bar } = this.state; instead of const { props: { foo }, state: { bar } } = this;
  • Avoid nested ternaries as they often result in code that is difficult to read. If you have logical branches in your code that are dependent on the value of a string, prefer using an object as a map to multiple values (when evaluation is trivial) or a switch statement (when evaluation is more complex, like when branching on which React component to return):
// don't do this
const foo = str == 'a' ? 123 : str === 'b' ? 456 : str === 'c' : 789 : 0;

// do this
const foo = {
  a: 123,
  b: 456,
  c: 789,
}[str] || 0;

// or do this
switch (str) {
  case 'a':
    return <ComponentA />;
  case 'b':
    return <ComponentB />;
  case 'c':
    return <ComponentC />;
  case 'd':
    return <ComponentD />;

If your nested ternaries are in the form of predicates evaluating to booleans, prefer an if/if-else/else statement that is siloed to a separate, pure function:

const foo = getFoo(a, b);

function getFoo(a, b, c) {
  if (a.includes("foo")) {
    return 123;
  } else if (a === b) {
    return 456;
  } else {
    return 0;
  • Be conservative with what comments you add to the codebase. Comments shouldn’t be used as reminders or as todos–record those by creating a new issue in Github. Ideally, code should be written in such a way that it explains itself clearly. When it does not, you should first try rewriting the code. If for whatever reason you are unable to write something clearly, add a comment to explain the “why”.

// don't do this--the comment is redundant

// get the native permissions for this db
const nativePermissions = getNativePermissions(perms, groupId, {

// don't add TODOs -- they quickly become forgotten cruft

isSearchable(): boolean {
  // TODO: this should return the thing instead
  return this.isString();

// this is acceptable -- the implementer explains a not-obvious edge case of a third party library

// foo-lib seems to return undefined/NaN occasionally, which breaks things
if (isNaN(x) || isNaN(y)) {

  • Avoid complex logical expressions inside of if statements
// don't do this
if (typeof children === "string" && children.split(/\n/g).length > 1) {
  // ...

// do this
const isMultilineText =
  typeof children === "string" && children.split(/\n/g).length > 1;
if (isMultilineText) {
  // ...
  • Use ALL_CAPS for constants
// do this
const MIN_HEIGHT = 200;

// also acceptable
  camelCaseProps: "are OK",
  abc: 123,
  • Prefer named exports over default exports
// this makes it harder to search for Widget
import Foo from "./Widget";
// do this to enforce using the proper name
import { Widget } from "./Widget";
  • Avoid magic strings and numbers
// don't do this
const options = _.times(10, () => ...);

// do this in a constants file
export const MAX_NUM_OPTIONS = 10;
const options = _.times(MAX_NUM_OPTIONS,  () => ...);

Write Declarative Code

You should write code with other engineers in mind as other engineers will spend more time reading than you spend writing (and re-writing). Code is more readable when it tells the computer “what to do” versus “how to do.” Avoid imperative patterns like for loops:

// don't do this
let foo = [];
for (let i = 0; i < list.length; i++) {
  if (list[i].bar === false) {


// do this
const foo = list.filter(entry => !== false);

When dealing with business logic you don’t want to be concerned with the specifics of the language. Instead of writing const query = new Question(card).query(); which entails instantiating a new Question instance and calling a query method on said instance, you should introduce a function like getQueryFromCard(card) so that implementers can avoid thinking about what goes into getting a query value from a card.

Component Styling Tree Rings

classic / global CSS with BEM style selectors (deprecated)

.Button.Button--primary {
  color: -var(--color-brand);

atomic / utility CSS (discouraged)

.text-brand {
  color: -var(--color-brand);
const Foo = () => <div className="text-brand" />;

inline style (discouraged)

const Foo = ({ color ) =>
  <div style={{ color: color }} />

CSS modules (deprecated)

:local(.primary) {
  color: -var(--color-brand);
import style from "./Foo.css";

const Foo = () => <div className={style.primary} />;


import styled from "@emotion/styled";

const Foo = styled.div`
  color: ${props => props.color};

const Bar = ({ color }) => <Foo color={color} />;


Popovers are popups or modals.

In Metabase core, they are visually responsive: they appear above or below the element that triggers their appearance. Their height is automatically calculated to make them fit on the screen.

Where to Find Popovers in the User Journey

When creating custom questions

  1. From home, click on New and then Question
  2. 👀 The option picker that automatically opened next to Pick your starting data is a <Popover />.
  3. Choose Sample Database if not already selected
  4. Choose any of the tables, for example People

Here, clicking on the following will open <Popover /> components:

  • Pick columns (arrow on the right-hand side of a FieldsPicker control in the section labeled Data)
  • Gray icon of a grid with + below section labeled Data
  • Add filters to narrow your answers
  • Pick the metric you want to see
  • Pick a column to group by
  • Sort icon with arrows pointing up and down above Visualize button

Unit testing

Setup pattern

We use the following pattern to unit test components:

import React from "react";
import userEvent from "@testing-library/user-event";
import { Collection } from "metabase-types/api";
import { createMockCollection } from "metabase-types/api/mocks";
import { renderWithProviders, screen } from "__support__/ui";
import CollectionHeader from "./CollectionHeader";

interface SetupOpts {
  collection: Collection;

const setup = ({ collection }: SetupOpts) => {
  const onUpdateCollection = jest.fn();


  return { onUpdateCollection };

describe("CollectionHeader", () => {
  it("should be able to update the name of the collection", () => {
    const collection = createMockCollection({
      name: "Old name",

    const { onUpdateCollection } = setup({

    await userEvent.clear(screen.getByDisplayValue("Old name"));
    await userEvent.type(screen.getByPlaceholderText("Add title"), "New title");

      name: "New name",

Key points:

  • setup function
  • renderWithProviders adds providers used by the app, including redux

Request mocking

We use fetch-mock to mock requests:

import fetchMock from "fetch-mock";
import { setupCollectionsEndpoints } from "__support__/server-mocks";

interface SetupOpts {
  collections: Collection[];

const setup = ({ collections }: SetupOpts) => {
  setupCollectionsEndpoints({ collections });

  // renderWithProviders and other setup

describe("Component", () => {
  it("renders correctly", async () => {
    expect(await screen.findByText("Collection")).toBeInTheDocument();

Key points:

  • setup function
  • Call helpers from __support__/server-mocks to setup endpoints for your data

Read docs for other versions of Metabase.

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