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Now creating users works,

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but at the moment, we can create the same user,

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with the same email address, multiple times.

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And that's typically not what we want to do.

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Therefore, to avoid that we do that,

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in our signup handler function here,

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we should check if a user for a given email already exists.

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So once we connected here to the database,

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before we hash the password and store the user,

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I want to check if a user already exists.

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I want to get a existing user by using

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db.collection('users') and calling find there,

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or findOne to be precise.

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FindOne allows us to find one document,

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and we pass a object to findOne

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where we describe how to search for that document.

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Key value pairs here,

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and the keys are the keys we have in our documents.

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So email, password, or underscore id.

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And we can then search for email,

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and the values are the values

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we want to search for in the database.

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So in this case, I want to search for documents,

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which already have the email we're getting here

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stored in the email field.

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Now we should await this, because that it returns a promise.

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And then it's either undefined,

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if we did not find a user for that email,

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or it is the user object.

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So if it is not undefined, if it's truthy,

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so if it is a object with that user data,

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then we know that the user already exists.

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So that's then when I want to return

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so that we don't execute the other code.

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And I want to send back a different response,

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a response with a status of still 422, maybe,

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and some data where I have a message where I say

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user exists already, or something like this.

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Now one thing we also want to do here,

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which I haven't done before,

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is close our database connection.

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We should always do that.

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So we should use that client

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and call close once we're done here,

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and also here at the bottom, of course.

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We want to do that.

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But now with those changes made, we should not be able

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to create the same user more than once.

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If I try to send this request,

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or the already existing email address again,

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I get a 422 error with the user exists already message.

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If I use a different email address

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which does not exist yet, it should work though.

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And it does.

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And in the database, if we refresh it,

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we indeed see that every email address exists only once.

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So that's now also working as it should.