Currently Cognito allows merging federated users (users logging from external identity providers like Google) to native users (users who signed up via username and password combination).
Is there a way to merge an existing federated user with a new native user?
No. You can only create a link from a user who has never signed in. That is, you can only link the new user at the point they are created (in the pre-auth trigger).
"This allows you to create a link from the existing user account to an external federated user identity that has not yet been used to sign in".
Personally I catch this case in the pre-signup trigger, and reject the sign up with a custom message ("An account already exists with this email address, please sign in using Google").
Here is my pre-signup lambda in case you find it useful
const AWS = require("aws-sdk");
const cognito = new AWS.CognitoIdentityServiceProvider();
exports.handler = (event, context, callback) => {
function checkForExistingUsers(event, linkToExistingUser) {
console.log("Executing checkForExistingUsers");
var params = {
UserPoolId: event.userPoolId,
AttributesToGet: ['sub', 'email'],
Filter: "email = \"" + event.request.userAttributes.email + "\""
};
return new Promise((resolve, reject) =>
cognito.listUsers(params, (err, result) => {
if (err) {
reject(err);
return;
}
if (result && result.Users && result.Users[0] && result.Users[0].Username && linkToExistingUser) {
console.log("Found existing users: ", result.Users);
if (result.Users.length > 1){
result.Users.sort((a, b) => (a.UserCreateDate > b.UserCreateDate) ? 1 : -1);
console.log("Found more than one existing users. Ordered by createdDate: ", result.Users);
}
linkUser(result.Users[0].Username, event).then(result => {
resolve(result);
})
.catch(error => {
reject(err);
return;
});
} else {
resolve(result);
}
})
);
}
function linkUser(sub, event) {
console.log("Linking user accounts with target sub: " + sub + "and event: ", event);
//By default, assume the existing account is a Cognito username/password
var destinationProvider = "Cognito";
var destinationSub = sub;
//If the existing user is in fact an external user (Xero etc), override the the provider
if (sub.includes("_")) {
destinationProvider = sub.split("_")[0];
destinationSub = sub.split("_")[1];
}
var params = {
DestinationUser: {
ProviderAttributeValue: destinationSub,
ProviderName: destinationProvider
},
SourceUser: {
ProviderAttributeName: 'Cognito_Subject',
ProviderAttributeValue: event.userName.split("_")[1],
ProviderName: event.userName.split("_")[0]
},
UserPoolId: event.userPoolId
};
console.log("Parameters for adminLinkProviderForUser: ", params);
return new Promise((resolve, reject) =>
cognito.adminLinkProviderForUser(params, (err, result) => {
if (err) {
console.log("Error encountered whilst linking users: ", err);
reject(err);
return;
}
console.log("Successfully linked users.");
resolve(result);
})
);
}
console.log(JSON.stringify(event));
if (event.triggerSource == "PreSignUp_SignUp" || event.triggerSource == "PreSignUp_AdminCreateUser") {
checkForExistingUsers(event, false).then(result => {
if (result != null && result.Users != null && result.Users[0] != null) {
console.log("Found at least one existing account with that email address: ", result);
console.log("Rejecting sign-up");
//prevent sign-up
callback("An external provider account alreadys exists for that email address", null);
} else {
//proceed with sign-up
callback(null, event);
}
})
.catch(error => {
console.log("Error checking for existing users: ", error);
//proceed with sign-up
callback(null, event);
});
}
if (event.triggerSource == "PreSignUp_ExternalProvider") {
checkForExistingUsers(event, true).then(result => {
console.log("Completed looking up users and linking them: ", result);
callback(null, event);
})
.catch(error => {
console.log("Error checking for existing users: ", error);
//proceed with sign-up
callback(null, event);
});
}
};