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Let's continue by adding a constraint for
our user model.

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By constraints, I mean rules for

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more in-depth checks performed at the SQL
level versus at the sequelize level.

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For example, earlier we use
the notEmpty validator to ensure that

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values don't get stored as empty strings.

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And the isEmail validator
to check the email format.

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As you learned,
if any of those validations fail,

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a SQL query will not be
sent to the database.

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However, with constraint checks
a SQL query is performed.

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But if the constraint check fails,
the database throws an error

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preventing the post or update, and
sequelize lets us know about it.

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This is how constraint checks
differ from standard validation.

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A common example is checking that the
email submitted is a unique email address.

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A user submits an entry and their email
gets checked against other emails

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in the database to
ensure that it's unique.

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In the user model, I can define
a unique constraint on the email field

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by adding the unique property to
the email object and setting it to true.

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Back in Postman, I'll attempt to create
a new user entry using an email that

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already exists in the users
table test@teamtreehouse.com.

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When I press Send, I get a 400 status and

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see the error email must
be unique in the response.

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And notice in the console that the error
log is SequelizeUniqueConstrainError.

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An attempt to insert an email
that already exists will throw

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a SequeleUniqueConstraintError,

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which prevents duplicate data from
being inserted into the database.

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Like validators, we can customize
the error message for unique constraints.

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Set the value of unique to an object,

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inside the object use the message property
and set it to your custom message.

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For example,
the email you entered already exists.

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I'll test sending the same post request,
and this time I receive my

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custom error message in the response.
Changing the email address

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successfully creates and stores the new
user account in the database.

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All right, I set up my one constraint for
unique user email addresses.

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Next, I'll continue by adding
two more user model validations.