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=== Performing CRUD Operations

Create, Read, Update and Delete (or CRUD) are the most common use cases when working with databases. The following examples will work you through how to perform each.

==== Create - Saving domain instances

To create and save a new instance to the database you should use the `save()` method:


[source,groovy]
----
new Book(title:"The Stand")
        .save()
        .toBlocking()
        .first()
----

The `save()` method returns an https://reactivex.io/RxJava/javadoc/rx/Observable.html[rx.Observable].

Notice that in this example we are calling `toBlocking()`, this is purely for demonstration and testing purposes to ensure the operation completes immediately.

In a real world application you would typically subscribe to the observable in order to receive a result:


[source,groovy]
----
new Book(title:"The Stand")
    .save()
    .subscribe { Book book ->
        println "Title = ${book.title}"
    }
----

NOTE: If you don't call `subscribe(..)` or convert the `Observable` into a blocking operation, then no query will be executed! At least one call to `subscribe(..)` is required.

==== Read - Retrieving domain instances

To retrieve an instance you can use the static `get(..)` method:

[source,groovy]
----
Book.get(id)
    .subscribe { Book book ->
        println "Title = ${book.title}"
    }
----

==== Update - Updating instances

To update an instance after retrieving it you can use the https://reactivex.io/RxJava/javadoc/rx/Observable.html#switchMap-rx.functions.Func1-[Observable.switchMap] method:

[source,groovy]
----
Book.get(id)
    .switchMap() { Book book ->
        book.title = "The Shining"
        book.save()
    }.subscribe { Book updated ->
        println "Book updated!"
    }
----

The `switchMap` transforms an `Observable` and converts the result into another `Observable`. However, this is not the most efficient way to perform updates as you can use https://docs.grails.org/latest/grails-data/hibernate5/manual/index.html#whereQueries[where queries] to update an instance without retrieving it:


[source,groovy]
----
Book.where { id == id }
    .updateAll(title: "The Shining")
    .subscribe { Number updateCount ->
        println "${updateCount} books were updated!"
    }
----

==== Delete - Deleting instances

To delete an instance after retreiving it you can use the https://reactivex.io/RxJava/javadoc/rx/Observable.html#switchMap-rx.functions.Func1-[Observable.switchMap] method:

[source,groovy]
----
Book.get(id)
    .switchMap() { Book book ->
        book.delete()
    }
    .subscribe { Boolean wasDeleted ->
        if(wasDeleted) {
            println "Book deleted!"
        }
    }
----

But, once again it is more efficient with where queries:

[source,groovy]
----
Book.where { id == id }
    .deleteAll()
    .subscribe { Number deleteCount ->
        println "${deleteCount} books were deleted!"
    }
----


