php-notes/Nested_Models.md

# Nesting Models

## Objectives

Currently, we only have two unrelated models: people and locations.  Let's add the functionality so that whenever we have JSON that represents a person, it will contain a `home` property, which is the associated `location` object:

```JSON
{
    "id":4,
    "name":"Bob",
    "age":25,
    "home": {
        "id":2,
        "street": "123 Fake Street",
        "city": "Aurora",
        "state": "NY"
    }
}
```

Also, when we have a location, it will contain an array of people objects:

```JSON
{
    "id":2,
    "street": "123 Fake Street",
    "city": "Aurora",
    "state": "NY",
    "inhabitants":[
        {
            "id":4,
            "name":"Bob",
            "age":25
        },        
        {
            "id":7,
            "name":"Sally",
            "age":74
        },        
    ]
}
```

This will be a one-to-many relationship

## Add home_id to people

First, go into `psql` and add a `home_id` INT column to the `people` table:

```sql
ALTER TABLE people ADD COLUMN home_id INT;
```

Now we can give people homes like so:

```sql
UPDATE people SET home_id = 2 WHERE id = 4;
```

The previous code will find the row with an `id` of 4 and set the `home_id` column to 2.

## Update People SQL

The first thing we want to do is to update the SQL code in the `People` class so that each row from the `people` table is joined onto a corresponding row from the `locations` table.  Once we have the additional columns for each row in the `people` table, we can use that information to create a home for each `Person` object.  Try the following in `psql`:

```SQL
SELECT * FROM people JOIN locations ON people.home_id = locations.id;
```

The above SQL statement will only show rows where the people have a value in the `home_id` column.  This is because Postgres is attempting to create rows by matching rows in the `people` table with rows in the `locations` table, based on whether `people.home_id = locations.id`.  When it encounters a row from the `people` table where the `home_id` column is `NULL`, it can't find any rows from the `locations` table to match it with, since no rows from `locations` have an `id` column of `NULL`.  Since no matches can be found, it doesn't include that row from `people`.  We need to perform the join and then add any missing rows from `people`:

```SQL
SELECT * FROM people LEFT JOIN locations ON people.home_id = locations.id;
```

Now that we have all the people rows, we're ready to plug that into our php.  Alter the `pg_query` code in our `People::all()` function:

```php
$results = pg_query("SELECT * FROM people LEFT JOIN locations ON people.home_id = locations.id");
```

If you view http://localhost:8888/people, you'll see something funky.  Some people have `NULL` ids, and other rows have unexpected `id` columns.  The reason for this can be discovered by looking back in `psql` at our previous `LEFT JOIN` statement results.  You'll notice there are two `id` columns, one for the `people` table and one for `locations` table.  When PHP attempts to convert a row into an object, when it reads the `people` table's `id` column, creates an `id` property for the object.  It then goes through, adding `name`, `age`, and `home_id` properties.  It then reaches the `id` column for the `locations` table and overwrites the `id` property on the `$row_object` with the value from the `id` column of the `locations` table.

To fix this, we can alter our SQL statement to rename one or both of the id columns.  Let's rename the `id` column from the `locations` table.

```SQL
SELECT
    people.*,
    locations.id AS location_id,
    locations.street,
    locations.city,   
    locations.state   
FROM people
LEFT JOIN locations
    ON people.home_id = locations.id;
```

Now if we change our SQL statement in PHP, we'll see better results:

```php
$results = pg_query("SELECT
    people.*,
    locations.id AS location_id,
    locations.street,
    locations.city,
    locations.state
FROM people
LEFT JOIN locations
    ON people.home_id = locations.id;");
```

## Give People Locations

If we put a `var_dump` inside the while loop in our `People::all()` function, we can see `$row_object` now contains information about each person's location (if you're using Postman to view http://localhost:8888/people, you might need to switch to the `Raw` view).

```php
while($row_object){
    var_dump($row_object); //print $row_object so we can see what it looks like now

    $new_person = new Person(
        $row_object->id,
        $row_object->name,
        $row_object->age
    );
    $people[] = $new_person;

    $row_object = pg_fetch_object($results);
}
```

Now lets use the additional information on `$row_object` to create Location objects and add them to `$new_person` where necessary.  First, let's include the `Location` model at the top of `models/person.php`.

```php
include_once __DIR__ . '/location.php';
```

Now, inside the `People::all()` `while` loop, we'll add the logic to create a location and add it to `$new_person`:

```php
$new_person = new Person(
    $row_object->id,
    $row_object->name,
    $row_object->age
);

if($row_object->location_id){ //test if location_id is truthy
    $new_location = new Location( //create a location from the row data
        intval($row_object->location_id),
        $row_object->street,
        $row_object->city,
        $row_object->state
    );
    $new_person->home = $new_location; //attach $new_location to $new_person->home
}

$people[] = $new_person;

$row_object = pg_fetch_object($results);
```

Here we test to see if `$row_object->location_id` is truthy.  If it has a value -- a string representation of the id column of `locations` table -- then the block of code belonging to the `if` statement will run, creating `$new_location` and attaching it to the `home` property of `$new_person`.