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We talked a lot about schemas, structures, relations.

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Now let's talk about schema validation

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and with that, I'll come back to the data type thing

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and also to our schema structure in general. You learned that mongodb is very flexible,

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you can have totally different schemas or documents in one and the same collection

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and that flexibility is a huge plus

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but sometimes you want to lock down your flexibility here, you want to get rid of it. Sometimes you need a strict

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schema because you know your application is going to fetch posts and it is going to access the title

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on each post and it does expect that each title is a string

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and for cases like this, schema validation can help you. Now what is schema validation?

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Let's say we have an insert operation going on on our collection, on our posts collection maybe and

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the same would be true for update or insertMany operations by the way,

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so we're trying to add or edit some data in a collection.

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If we add a validation schema and I will show you how you add one in the next lecture, if we added one,

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then the schema will validate or the mongodb will validate the incoming data based on the schema we

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defined and either it accepts it and then allows the write to the database or it rejects the incoming data,

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hence your database is not touched and is not changed and the user gets an error.

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To be precise, you can define schema validation in the way I'll show it in the next lecture and you can also define

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which kinds of operations you want to validate and what you want to do if validation fails

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with two settings you can add.

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You can define which document should get validated

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and what happens if it fails,

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you can either set the validation level to strict which means all inserts and updates are checked or

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to moderate

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which means all inserts are checked but updates are only checked for documents which were valid before.

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So if you had some invalid data in there because it existed before you set up a schema, then you could still

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change these documents even if they don't fit your schema. For validation action,

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you can decide whether you want to throw an error and not go on with the insert or update, so it will

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not change your database data or if you only want to log a warning and proceed,

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so then you would still write the data, you would still change the data but log a warning that it did not

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fulfill your criteria.

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And what you want to choose here really depends on your application,

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if you're ok with incorrect data ending up in your database, you just want to be aware of it or if you

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totally need to block it. Let's have a look at that in code next.