[#MetaDataProduction]
include::../variables.adoc[]

The first thing that an application built using QQQ needs to do is to define its meta data.
This basically means the construction of a `QInstance` object, which is populated with the
meta data objects defining the backend(s), tables, processes, possible-value sources, joins,
authentication provider, etc, that make your application.

There are various styles that can be used for how you define your meta data, and for the
most part they can be mixed and matched.  They will be presented here based on the historical
evolution of how they were added to QQQ, where we generally believe that better techniques have
been added over time.  So, you may wish to skip the earlier techniques, and jump straight to
the end of this section.  However, it can always be instructive to learn about the past, so,
read at your own pace.

== Omni-Provider
At the most basic level, the way to populate a `QInstance` is the simple and direct approach of creating
one big class, possibly with just one big method, and just doing all the work directly inline there.

This may (clearly) violate several good engineering principles.  However, it does have the benefit of
being simple - if all of your meta-data is defined in one place, it can be pretty simple to find where
that place is.  So - especially in a small project, this technique may be worth continuing to consider.

Re: "doing all the work" as mentioned above - what work are we talking about?  At a minimum, we need
to construct the following meta-data objects, and pass them into our `QInstance`:

* `QAuthenticationMetaData` - how (or if!) users will be authenticated to the application.
* `QBackendMeataData` - a backend data store.
* `QTableMetaData` - a table (within the backend).
* `QAppMetaData` - an organizational unit to present the other elements in a UI.

Here's what a single-method omni-provider could look like:

[source,java]
.About the simplest possible single-file meta-data provider
----
public QInstance defineQInstance()
{
    QInstance qInstance = new QInstance();

    qInstance.setAuthentication(new QAuthenticationMetaData()
       .withName("anonymous")
       .withType(QAuthenticationType.FULLY_ANONYMOUS));

    qInstance.addBackend(new QBackendMetaData()
       .withBackendType(MemoryBackendModule.class)
       .withName("memoryBackend"));

    qInstance.addTable(new QTableMetaData()
        .withName("myTable")
        .withPrimaryKeyField("id")
        .withBackendName("memoryBackend")
        .withField(new QFieldMetaData("id", QFieldType.INTEGER)));

    qInstance.addApp(new QAppMetaData()
       .withName("myApp")
       .withSectionOfChildren(new QAppSection().withName("mySection"),
          qInstance.getTable("myTable")))

   return (qInstance);
}
----

== Multi-method Omni-Provider

The next evolution of meta-data production comes by just applying some basic better-engineering
principles, and splitting up from a single method that constructs all the things, to at least
using unique methods to construct each thing, then calling those methods to add their results
to the QInstance.

[source,java]
.Multi-method omni- meta-data provider
----
public QInstance defineQInstance()
{
    QInstance qInstance = new QInstance();
    qInstance.setAuthentication(defineAuthenticationMetaData());
    qInstance.addBackend(defineBackendMetaData());
    qInstance.addTable(defineMyTableMetaData());
    qInstance.addApp(defineMyAppMetaData(qInstance));
    return qInstance;
}

public QAuthenticationMetaData defineAuthenticationMetaData()
{
    return new QAuthenticationMetaData()
       .withName("anonymous")
       .withType(QAuthenticationType.FULLY_ANONYMOUS);
}

public QBackendMetaData defineBackendMetaData()
{
    return new QBackendMetaData()
       .withBackendType(MemoryBackendModule.class)
       .withName("memoryBackend");
}

// implementations of defineMyTableMetaData() and defineMyAppMetaData(qInstance)
// left as an exercise for the reader
----

== Multi-class Providers

Then the next logical evolution would be to put each of these single meta-data producing
objects into its own class, along with calls to those classes.  This gets us away from the
"5000 line" single-class, and lets us stop using the word "omni":

[source,java]
.Multi-class meta-data providers
----
public QInstance defineQInstance()
{
    QInstance qInstance = new QInstance();
    qInstance.setAuthentication(new AuthMetaDataProvider().defineAuthenticationMetaData());
    qInstance.addBackend(new BackendMetaDataProvider().defineBackendMetaData());
    qInstance.addTable(new MyTableMetaDataProvider().defineTableMetaData());
    qInstance.addApp(new MyAppMetaDataProvider().defineAppMetaData(qInstance));
    return qInstance;
}

public class AuthMetaDataProvider
{
    public QAuthenticationMetaData defineAuthenticationMetaData()
    {
        return new QAuthenticationMetaData()
           .withName("anonymous")
           .withType(QAuthenticationType.FULLY_ANONYMOUS);
    }
}

public class BackendMetaDataProvider
{
    public QBackendMetaData defineBackendMetaData()
    {
        return new QBackendMetaData()
           .withBackendType(MemoryBackendModule.class)
           .withName("memoryBackend");
    }
}

// implementations of MyTableMetaDataProvider and MyAppMetaDataProvider
// left as an exercise for the reader
----

== MetaDataProducerInterface

As the size of your application grows, if you're doing per-object meta-data providers, you may find it
burdensome, when adding a new object to your instance, to have to write code for it in two places -
that is - a new class to produce that meta-data object, AND a single line of code to add that object
to your `QInstance`.  As such, a mechanism exists to let you avoid that line-of-code for adding the object
to the `QInstance`.

This mechanism involves adding the `MetaDataProducerInterface` to all of your classes that produce a
meta-data object.  This interface is generic, with a type parameter that will typically be the type of
meta-data object you are producing, such as `QTableMetaData`, `QProcessMetaData`, or `QWidgetMetaData`,
(technically, any class which implements `TopLevelMetaData`).  Implementers of the interface are then
required to override just one method: `T produce(QInstance qInstance) throws QException;`

Once you have your `MetaDataProducerInterface` classes defined, then there's a one-time call needed
to add all of the objects produced by these classes to your `QInstance` - as shown here:

[source,java]
.Using MetaDataProducerInterface
----
public QInstance defineQInstance()
{
    QInstance qInstance = new QInstance();
    MetaDataProducerHelper.processAllMetaDataProducersInPackage(qInstance,
      "com.mydomain.myapplication");
    return qInstance;
}

public class AuthMetaDataProducer implements MetaDataProducerInterface<QAuthenticationMetaData>
{
    @Override
    public QAuthenticationMetaData produce(QInstance qInstance)
    {
        return new QAuthenticationMetaData()
           .withName("anonymous")
           .withType(QAuthenticationType.FULLY_ANONYMOUS);
    }
}

public class BackendMetaDataProducer implements MetaDataProducerInterface<QBackendMetaData>
{
    @Override
    public QBackendMetaData defineBackendMetaData()
    {
        return new QBackendMetaData()
           .withBackendType(MemoryBackendModule.class)
           .withName("memoryBackend");
    }
}

// implementations of MyTableMetaDataProvider and MyAppMetaDataProvider
// left as an exercise for the reader
----

=== MetaDataProducerMultiOutput
It is worth mentioning, that sometimes it might feel like a bridge that's a bit too far, to make
every single one of your meta-data objects require its own class.  Some may argue that it's best
to do it that way - single responsibility principle, etc.  But, if you're producing, say, 5 widgets
that are all related, and it's only a handful of lines of code for each one, maybe you'd rather
produce them all in the same class.  Or maybe when you define a table, you'd like to define its
joins and widgets at the same time.

This approach can be accomplished by making the type argument for your `MetaDataProducerInterface` be
`MetaDataProducerMultiOutput` - a simple class that just wraps a list of other `MetaDataProducerOutput`
objects.

[source,java]
.Returning a MetaDataProducerMultiOutput
----
public class MyMultiProducer implements MetaDataProducerInterface<MetaDataProducerMultiOutput>
{
    @Override
    public MetaDataProducerMultiOutput produce(QInstance qInstance)
    {
       MetaDataProducerMultiOutput output = new MetaDataProducerMultiOutput();

       output.add(new QPossibleValueSource()...);
       output.add(new QJoinMetaData()...);
       output.add(new QJoinMetaData()...);
       output.add(new QWidgetMetaData()...);
       output.add(new QTableMetaData()...);

       return (output);
    }
}
----


== Aside: TableMetaData with RecordEntities
At this point, let's take a brief aside to dig deeper into the creation of a `QTableMeta` object.
Tables, being probably the most important meta-data type in QQQ, have a lot of information that can
be specified in their meta-data object.

At the same time, if you're writing any custom code in your QQQ application
(e.g., any processes or table customizers), where you're working with records from tables, you may
prefer being able to work with entity beans (e.g., java classes with typed getter & setter methods),
rather than the default object type that QQQ's ORM actions return, the `QRecord`, which carries all
of its values in a `Map` (where you don't get compile-time checks of field names or data types).
QQQ has a mechanism for dealing with this - in the form of the `QRecordEntity` class.

So - if you want to build your application using entity beans (which is recommended, for the compile-time
safety that they provide in custom code), you will be writing a `QRecordEntity` class for each of your tables,
which will look like:

[source,java]
.QRecordEntity example
----
public class MyTable extends QRecordEntity
{
   public static final String TABLE_NAME = "myTable";

   @QField(isEditable = false, isPrimaryKey = true)
   private Integer id;

   @QField()
   private String name;

   // no-arg constructor and constructor that takes a QRecord
   // getters & setters (and optional fluent setters)
}
----

The point of introducing this topic here and now is, that a `QRecordEntity` can be used to shortcut to
defining some of the attributes in a `QTableMetaData` object.  Specifically, in a `MetaDataProducer<QTableMetaData>`
you may say:

[source,java]
.QTableMetaDataProducer using a QRecordEntity
----
public QTableMetaData produce(QInstance qInstance) throws QExcpetion
{
  return new QTableMetaData()
     .withName(MyTable.TABLE_NAME)
     .withFieldsFromEntity(MyTable.class)
     .withBackendName("memoryBackend");
}
----

That `withFieldsFromEntity` call is one of the biggest benefits of this technique.  It allows you to avoid defining
all of the fields in you table in two places (the entity and the table meta-data).

== MetaData Producing Annotations for Entities

If you are using `QRecordEntity` classes that correspond to your tables, then you can take advantage of some
additional annotations on those classes, to produce more related meta-data objects associated with those tables.
The point of this is to eliminate boilerplate, and simplify / speed up the process of getting a new table
built and deployed in your application, with some bells and whistles added.

=== @QMetaDataProducingEntity
This is an annotation to go on a QRecordEntity class, which you would like to be
processed by `MetaDataProducerHelper`, to automatically produce some meta-data
objects.  Specifically supports:

* Making a possible-value-source out of the table.
* Processing child tables to create joins and childRecordList widgets

=== @ChildTable
This is an annotation used as a value that goes inside a `@QMetadataProducingEntity` annotation, to define
child-tables, e.g., for producing joins and childRecordList widgets related to the table defined in the entity class.

=== @ChildJoin
This is an annotation used as a value inside a `@ChildTable` inside a `@QMetadataProducingEntity` annotation,
to control the generation of a `QJoinMetaData`, as a `ONE_TO_MANY` type join from the table represented by
the annotated entity, to the table referenced in the `@ChildTable` annotation.

=== @ChildRecordListWidget
This is an annotation used as a value that goes inside a `@QMetadataProducingEntity` annotation, to control
the generation of a QWidgetMetaData - for a ChildRecordList widget.

[source,java]
.QRecordEntity with meta-data producing annotations
----
@QMetaDataProducingEntity(
   producePossibleValueSource = true,
   childTables = {
      @ChildTable(
         childTableEntityClass = MyChildTable.class,
         childJoin = @ChildJoin(enabled = true),
         childRecordListWidget = @ChildRecordListWidget(enabled = true, label = "Children"))
   }
)
public class MyTable extends QRecordEntity
{
   public static final String TABLE_NAME = "myTable";
   // class body left as exercise for reader
}
----

The class given in the example above, if processed by the `MetaDataProducerHelper`, would add the following
meta-data objects to your `QInstance`:

* A `QPossibleValueSource` named `myTable`, of type `TABLE`, with `myTable` as its backing table.
* A `QJoinMetaData` named `myTableJoinMyChildTable`, as a `ONE_TO_MANY` type, between those two tables.
* A `QWidgetMetaData` named `myTableJoinMyChildTable`, as a `CHILD_RECORD_LIST` type, that will show a list of
records from `myChildTable` as a widget, when viewing a record from `myTable`.

== Other MetaData Producing Annotations

Similar to these annotations for a `RecordEntity`, a similar one exists for a `PossibleValueEnum` class,
to automatically write the meta-data to use that enum as a possible value source in your application:

=== @QMetaDataProducingPossibleValueEnum
This is an annotation to go on a `PossibleValueEnum` class, which you would like to be
processed by MetaDataProducerHelper, to automatically produce a PossibleValueSource meta-data
based on the enum.

[source,java]
.PossibleValueEnum with meta-data producing annotation
----
@QMetaDataProducingPossibleValueEnum(producePossibleValueSource = true)
public enum MyOptionsEnum implements PossibleValueEnum<Integer>
{
   // values and methods left as exercise for reader
}
----
The enum given in the example above, if processed by the `MetaDataProducerHelper`, would add the following
meta-data object to your `QInstance`:

* A `QPossibleValueSource` named `MyOptionsEnum`, of type `ENUM`, with `MyOptionsEnum` as its backing enum.
