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Instructor: Welcome back.

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We have the Todo bean.

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Let's say we have a Todo table in the database.

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JPA allows us to map your bean to the database.

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The way you can do that

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is by adding a simple annotation called @Entity.

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If you say the Todo is an entity

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it means that this bean is mapped to a database table.

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Let's just add in entity, Ctrl + 1,

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input jakarta.persistence.

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So you'd see that jakarta.persistence.Entity is imported in.

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Whenever you have a entity

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you would also need to have a ID.

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So you need to define primary key.

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I'll add in @Id and say Ctrl + 1,

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import ID jakarta.persistence.

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Let's do that.

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And we'd also want to generate this using a sequence.

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So I'll say @GeneratedValue.

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So think about what we are doing in here.

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We are mapping Todo bean, this bean,

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to a table in the database.

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That's what entities allow you Todo.

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It allows you to map a bean to a database table.

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One of the magic of Spring Boot auto-configuration

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is that if it sees any entities

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then it would automatically start creating tables in H2.

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If you look at our pom.xml, we have H2 database in here.

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So let's see where it is.

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If you look at pom.xml, we have H2 database in here.

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And as soon as Spring Boot sees H2 database

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what it does is it would start pre-configuring it.

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One of the pre-configuring steps is if it sees any entities,

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if it sees any entities like this

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it would directly start creating tables for them.

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Let's see that in action. Let's go to H2 console.

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Let's refresh. Let's re-log in by connecting.

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Make sure that you're using the right JDBC URL

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and say connect.

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Now you can see that there is a Todo table in here.

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So what's happening in here

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is that Spring Boot auto-configuration, whenever it sees H2

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or any in-memory database in the class path

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it'll directly create tables for all the entities.

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When you're defining your entity

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you have a lot of flexibility.

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Let's say the name of the table I would want to map to

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is not Todo, but let's say I would want to map

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to something else.

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So I would say name is equal to TodoABC and save.

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Let's refresh this.

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Continue. Connect.

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You can see that the name of the table is now TodoABC.

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So we are mapping to a table called TodoABC.

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By default it'll use the class name itself.

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I'll remove this.

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You also have flexibility

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in terms of customizing the column names as well.

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Instead of calling this USERNAME,

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let's say I would want to give it a different name.

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I can say column and give it a name.

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A name is equal to name.

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Right now if you look at this, it's called USERNAME.

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But if I save this and come back to H2 console,

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refresh it and connect it back,

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you would see that the name is now name.

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So this field is now mapped to this.

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JPA also supports mapping complex relationships.

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One-to-one, one-to-many, many-to-one,

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many-to-many and things like that.

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For now, let's remove the column.

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So I'll remove this column name is equal to name.

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I would want to continue using the default values

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which are generated.

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Make sure that the entity name is also Todo and save it out.

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And when I actually refresh this, go back and say connect,

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I should see Todo table in here with the usual column names.

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ID, DESCRIPTION, DONE, TARGET_DATE, USERNAME.

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So these look good.

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One thing you would observe is TARGET_DATE.

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The reason is because there is a init cap in here.

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So this is in caps.

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So what happens is in a database typically

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we separate words using underscore.

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We capitalized D in here,

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that's why the column name is created as TARGET_DATE.

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Let's organize imports and save this.

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Now I would want to populate some data to start off.

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So how can I populate some data to start off?

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I can create a file called data.sql.

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So src/main/resources, new, SQL file, data.sql.

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In the data.sql I can create queries

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and these queries will be executed at startup.

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So I'll say insert into todos.

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Actually, insert into todo.

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Values, and you can specify the values in here.

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The default GeneratedValues start with one,

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so I'll use 10,001 as the ID.

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The second one is DESCRIPTION.

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I'll say learn, or you can say Get AWS Certified.

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The next one is DONE, which is false.

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The order of these does not look good.

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So what I do is I would define my own columns in here.

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So I would want to insert in the right order.

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So I would want to define my own thing in here.

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So I would say ID, first ID, then DESCRIPTION,

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then I would want to insert...

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Actually, before DESCRIPTION we need to have the USERNAME.

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ID, USERNAME, DESCRIPTION,

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after that would be the TARGET_DATE.

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And the last one is DONE.

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So let's insert values in the same way.

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So let's maximize this.

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And 10,001, username is in28minutes or Ranga.

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You can insert for any of those users

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and use that specific user.

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Get AWS certified.

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The target date, let's use a H2 function

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called current date.

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And is it done? It's false.

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And save this.

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If you go to console right now, you'd see an error.

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It's saying table TODO not found.

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By default data.sql is executed

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before the entities are processed.

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Tables are created when the entities are processed.

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So data.sql is executed before the table is created

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and we would want to change that.

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How do we change that?

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That's where we can make a change

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in the application.properties.

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That's something we can configure in application.properties.

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So let's open up application.properties

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and type in spring.jpa.defer-datasource-initialization.

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Make sure that you get the spelling right.

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It's spring.jpa.defer-datasource-initialization

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is equal to true, and say save.

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When I stopped and started the application again

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I saw a syntax exception.

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It says column in28minutes not found.

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The reason why it's thrown an error

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is because in data.sql you should use single quotes.

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So let me replace all these with single quotes.

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So instead of double quotes, let's use single quotes.

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And save this.

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Let's see it in action now, let's start it again.

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And now, everything looks hunky dory.

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The application has started up fine.

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Let's go to H2 console, refresh and connect again.

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TODO table, do you have the data?

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Yep, we have the data in here.

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Let's add a few more todos to data.sql.

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Ctrl + A. So let's add a few more ID.

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Should be increased. You cannot have duplicate IDs.

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So 10,003, 10,004.

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I'll add all the todos for in28minutes itself.

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We'll be making use of in28minutes ID.

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If you want, you can make use of the Ranga ID as well.

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So let's say get Azure certified, GCP certified,

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and let's say learn full stack or DevOps,

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whatever you'd want to do.

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Or learn to dance, right?

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So whatever you'd want to do, you can put them in here.

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Make sure that the ideas are unique and save them.

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And go back and refresh, connect, and TODO, run.

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One thing you need to remember

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is that even H2 is an in-memory database.

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When you restart the application,

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all the data in H2 will be lost.

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In the step, we got the H2 database ready.

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We have the table ready, we have the data ready

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and now we would want to display these todos

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connecting to the H2 database.

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We'd want to use JPA, connect to the database,

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and show the todos in here.

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And we would want to use JPA

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to perform all the operations as well.

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Delete, update, add, and a wide variety of them.

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Let's get started with that starting the next step.