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Schema

To set up a lambda function, first you need to define it on your GraphQL schema by using the @lambda directive. For example, to define a lambda function for the rank and bio fields in Author: 
type Author {
  id: ID!
  name: String! @search(by: [hash, trigram])
  dob: DateTime @search
  reputation: Float @search
  bio: String @lambda
  rank: Int @lambda
  isMe: Boolean @lambda
}
You can also define @lambda fields on interfaces, as follows: 
interface Character {
  id: ID!
  name: String! @search(by: [exact])
  bio: String @lambda
}

type Human implements Character {
  totalCredits: Float
}

type Droid implements Character {
  primaryFunction: String
}

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Resolvers

After the schema is ready, you can define your JavaScript mutation function and add it as a resolver in your JS source code. To add the resolver you can use either the addGraphQLResolvers or addMultiParentGraphQLResolvers methods.
A Lambda Field resolver can use a combination of parents, parent, dql, or graphql inside the function.
This example uses parent for the resolver function. You can find additional resolver examples using dql in the Lambda queries article, and using graphql in the Lambda mutations article.
For example, to define JavaScript lambda functions for
  • Author
  • Character
  • Human
  • Droid
and add them as resolvers, do the following 
const authorBio = ({ parent: { name, dob } }) =>
  `My name is ${name} and I was born on ${dob}.`
const characterBio = ({ parent: { name } }) => `My name is ${name}.`
const humanBio = ({ parent: { name, totalCredits } }) =>
  `My name is ${name}. I have ${totalCredits} credits.`
const droidBio = ({ parent: { name, primaryFunction } }) =>
  `My name is ${name}. My primary function is ${primaryFunction}.`

self.addGraphQLResolvers({
  "Author.bio": authorBio,
  "Character.bio": characterBio,
  "Human.bio": humanBio,
  "Droid.bio": droidBio,
})
For example, you can add a resolver for rank using a graphql call, as follows: 
async function rank({ parents }) {
  const idRepList = parents.map(function (parent) {
    return { id: parent.id, rep: parent.reputation }
  })
  const idRepMap = {}
  idRepList
    .sort((a, b) => (a.rep > b.rep ? -1 : 1))
    .forEach((a, i) => (idRepMap[a.id] = i + 1))
  return parents.map((p) => idRepMap[p.id])
}

self.addMultiParentGraphQLResolvers({
  "Author.rank": rank,
})
The following example demonstrates using the client-provided JWT to return true if the custom claim for USER from the JWT matches the id of the Author. 
async function isMe({ parent, authHeader }) {
  if (!authHeader) return false
  if (!authHeader.value) return false
  const headerValue = authHeader.value
  if (headerValue === "") return false
  const base64Url = headerValue.split(".")[1]
  const base = base64Url.replace(/-/g, "+").replace(/_/g, "/")
  const allClaims = JSON.parse(atob(base64))
  if (!allClaims["https://my.app.io/jwt/claims"]) return false
  const customClaims = allClaims["https://my.app.io/jwt/claims"]
  return customClaims.USER === parent.id
}

self.addGraphQLResolvers({
  "Author.isMe": isMe,
})

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Example

For example, if you execute the following GraphQL query: 
query {
  queryAuthor {
    name
    bio
    rank
    isMe
  }
}
You should see a response such as the following: 
{
  "queryAuthor": [
    {
      "name": "Ann Author",
      "bio": "My name is Ann Author and I was born on 2000-01-01T00:00:00Z.",
      "rank": 3,
      "isMe": false
    }
  ]
}
In the same way, if you execute the following GraphQL query on the Character interface: 
query {
  queryCharacter {
    name
    bio
  }
}
You should see a response such as the following: 
{
  "queryCharacter": [
    {
      "name": "Han",
      "bio": "My name is Han."
    },
    {
      "name": "R2-D2",
      "bio": "My name is R2-D2."
    }
  ]
}
The Human and Droid types inherit the bio lambda field from the Character interface.
For example, if you execute a queryHuman query with a selection set containing bio, then the lambda function registered for Human.bio is executed, as follows: 
query {
  queryHuman {
    name
    bio
  }
}
This query generates the following response: 
{
  "queryHuman": [
    {
      "name": "Han",
      "bio": "My name is Han. I have 10 credits."
    }
  ]
}