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loadOne

Similar to DataLoader's load method, uses the given callback function to read a single result from your business logic layer. To load a list, see loadMany.

Enhancements over DataLoader​

Thanks to the planning system in Grafast, loadOne can expose features that are not possible in DataLoader.

Attribute and parameter tracking​

A loadOne step (technically a LoadedRecordStep) keeps track of the attribute names accessed via .get(attrName) and any parameters set via .setParam(key, value). This information will be passed through to your callback function such that you may make more optimal calls to your backend business logic, only retrieving the data you need.

Input/output equivalence​

If you (optionally) pass an ioEquivalence parameter to loadOne (the second parameter) then you can use it to indicate which field(s) on the output is equivalent to the input(s). This enables an optimization where a chained fetch can instead be performed in parallel if the child only depends on an output which is equivalent to an input. Hopefully an example will make this clearer...

Imagine you're loading a user and their organization:

{
  currentUser {
    id
    name
    friends {
      id
      name
    }
  }
}

You might have plan resolvers such as:

const plans = {
  Query: {
    currentUser() {
      const $currentUserId = context().get("userId");
      return loadOne($currentUserId, batchGetUserById);
    },
  },
  User: {
    friends($user) {
      const $userId = $user.get("id");
      return loadMany($userId, batchGetFriendsByUserId);
    },
  },
};

In its current state the system doesn't know that the $user.get("id") is equivalent to the context().get("userId"), so this would result in a chained fetch:

stateDiagram direction LR state "batchGetUserById" as currentUser state "batchGetFriendsByUserId" as friends [*] --> currentUser currentUser --> friends

However, we can indicate that the output of the loadOne step's id property ($user.get("id")) is equivalent to its input (context().get("userId")):

 const plans = {
   Query: {
     currentUser() {
       const $currentUserId = context().get("userId");
-      return loadOne($currentUserId, batchGetUserById);
+      return loadOne($currentUserId, 'id', batchGetUserById);
     },
   },
   User: {
     friends($user) {
       const $userId = $user.get("id");
       return loadMany($userId, batchGetFriendsByUserId);
     },
   },
 };

Now the access to $user.get("id") will be equivalent to context().get("userId") - we no longer need to wait for the $user to load in order to fetch the friends:

stateDiagram direction LR state "batchGetUserById" as currentUser state "batchGetFriendsByUserId" as friends [*] --> currentUser [*] --> friends

Usage​

loadOne($spec, [$unaryStep,] [ioEquivalence,] callback)

loadOne accepts two to four arguments. The first is the step that specifies which records to load (the specifier step), the last is the callback function called with these specs responsible for loading them.

// Basic usage:
const $record = loadOne($spec, callback);

// Advanced usage:
const $record = loadOne($spec, $unaryStep, ioEquivalence, callback);
const $record = loadOne($spec, $unaryStep, callback);
const $record = loadOne($spec, ioEquivalence, callback);

Where:

  • $spec is any step
  • $unaryStep is any unary step - see Unary step usage below
  • ioEquivalence is either null, a string, an array of strings, or a string-string object map - see ioEquivalence usage below
  • and callback is a callback function responsible for fetching the data.

Callback​

The callback function is called with two arguments, the first is a list of the values from the specifier step $spec and the second is options that may affect the fetching of the records.

function callback(
  specs: ReadonlyArray<unknown>,
  options: {
    unary: unknown;
    attributes: ReadonlyArray<string>;
    params: Record<string, unknown>;
  },
): PromiseOrDirect<ReadonlyArray<unknown>>;
tip

For optimal results, we strongly recommend that the callback function is defined in a common location so that it can be reused over and over again, rather than defined inline. This will allow the underlying steps to optimize calls to this function.

Within this definition of callback:

  • specs is the runtime values of each value that $spec represented
  • options is an object containing:
    • unary: the runtime value that $unaryStep (if any) represented
    • attributes: the list of keys that have been accessed via $record.get('<key>')
    • params: the params set via $record.setParam('<key>', <value>)

specs is deduplicated using strict equality; so it is best to keep $spec simple - typically it should only represent a single scalar value - which is why $unaryStep exists.

options.unary is very useful to keep specs simple (so that fetch deduplication can work optimally) whilst passing in global values that you may need such as a database or API client.

options.attributes is useful for optimizing your fetch - e.g. if the user only ever requested $record.get('id') and $record.get('avatarUrl') then there's no need to fetch all the other attributes from your datasource.

options.params can be used to pass additional context to your callback function, perhaps options like "should we include archived records" or "should we expand 'customer' into a full object rather than just returning the identifier".

Basic usage​

const $userId = $post.get("author_id");
const $user = loadOne($userId, batchGetUserById);
// OR: const $user = loadOne($userId, 'id', batchGetUserById);

An example of the callback function might be:

async function batchGetUserById(ids, { attributes }) {
  // Your business logic would be called here; e.g. this might be the same
  // function that your DataLoaders would call, except we can pass additional
  // information to it.

  // For example, load from the database
  const rows = await db.query(
    sql`SELECT id, ${columnsToSql(attributes)} FROM users WHERE id = ANY($1);`,
    [ids],
  );

  // Ensure you return the same number of results, and in the same order!
  return ids.map((id) => rows.find((row) => row.id === id));
}

Unary step usage​

info

A unary step is a step that only ever represents one value, e.g. simple derivatives of context(), fieldArgs, or constant().

In addition to the forms seen in "Basic usage" above, you can pass a second step to loadOne. This second step must be a unary step, meaning that it must represent exactly one value across the entire request (not a batch of values like most steps).

const $userId = $post.get("author_id");
const $dbClient = context().get("dbClient");
const $user = loadOne($userId, $dbClient, "id", batchGetUserFromDbById);
// OR: const $user = loadOne($userId, $dbClient, batchGetUserFromDbById);

Since we know it will have exactly one value, we can pass it into the callback as a single value and our callback will be able to use it directly without having to perform any manual grouping.

This unary dependency is useful for fixed values (for example, those from GraphQL field arguments) and values on the GraphQL context such as clients to various APIs and other data sources.

An example of the callback function might be:

async function batchGetUserFromDbById(ids, { attributes, unary }) {
  const dbClient = unary;

  const rows = await dbClient.query(
    sql`SELECT id, ${columnsToSql(attributes)} FROM users WHERE id = ANY($1);`,
    [ids],
  );

  return ids.map((id) => rows.find((row) => row.id === id));
}

ioEquivalence usage​

The ioEquivalence optional parameter can accept the following values:

  • null to indicate no input/output equivalence
  • a string to indicate that the same named property on the output is equivalent to the entire input plan
  • if the step is a list() (or similar) plan, an array containing a list of keys (or null for no relation) on the output that are equivalent to the same entry in the input
  • if the step is a object() (or similar) plan, an object that maps between the attributes of the object and the key(s) in the output that are equivalent to the given entry on the input
Example for a list step
const $member = loadOne(
  list([$organizationId, $userId]),
  ["organization_id", "user_id"],
  batchGetMemberByOrganizationIdAndUserId,
);

// - batchGetMemberByOrganizationIdAndUserId will be called with a list of
//   2-tuples, the first value in each tuple being the organizationId and the
//   second the userId.
// - Due to the io equivalence (2nd argument):
//   - `$member.get("organization_id")` will return `$organizationId` directly
//   - `$member.get("user_id")` will return `$userId` directly
Example for an object step
const $member = loadOne(
  object({ oid: $organizationId, uid: $userId }),
  { oid: "organization_id", uid: "user_id" },
  batchGetMemberByOrganizationIdAndUserId,
);

// - batchGetMemberByOrganizationIdAndUserId will be called with a list of
//   objects; each object will have the key `oid` set to an organization id,
//   and the key `uid` set to the user ID.
// - Due to the io equivalence (2nd argument):
//   - `$member.get("organization_id")` will return the step used for `oid`
//     (i.e. `$organizationId`) directly
//   - Similarly `$member.get("user_id")` will return `$userId` directly

Passing multiple steps​

The list() or object() step can be used if you need to pass the value of more than one step into your callback:

const $isAdmin = $user.get("admin");
const $stripeId = $customer.get("stripe_id");
const $last4 = loadOne(list([$isAdmin, $stripeId]), getLast4FromStripeIfAdmin);

The first argument to the getLast4FromStripeIfAdmin callback will then be an array of all the tuples of values from these plans: ReadonlyArray<readonly [isAdmin: boolean, stripeId: string]>. The callback might look something like:

async function getLast4FromStripeIfAdmin(tuples) {
  const stripeIds = uniq(
    tuples
      .filter(([isAdmin, stripeId]) => isAdmin)
      .map(([isAdmin, stripeId]) => stripeId),
  );
  const last4s = await getLast4FromStripeIds(stripeIds);

  return tuples.map(([isAdmin, stripeId]) => {
    if (!isAdmin) return null;
    const index = stripeIds.indexOf(stripeId);
    return last4s[index];
  });
}

This technique can also be used with the unary step in advanced usage.

Performance impact from using list/object

Using list() / object() like this will likely reduce the effectiveness of loadOne's built in deduplication; to address this a stable object/list is required - please track this issue: https://github.com/graphile/crystal/issues/2170