Calculated fields in database - Laravel - laravel

Question

What is the best practice to store calculated fields in database.
For example, lets say a table has fields height, weight, bmi
A user enters height weight values and bmi field is automatically filled. How to achieve this with a form.
Formula for bmi
$bmi = weight / (height * height)
Tried the following
Profile Model
protected $table = 'profiles';
protected $fillable = ['user_id', 'weight', 'height', 'dob', 'age', 'bmi'];
public function user(){
return $this->belongsTo(User::class, 'user_id');
}
protected static function boot() {
parent::boot();
static::saving(function($model){
$model->bmi = $model->weight / ($model->height * $model->height);
$model->age = (date('Y') - date('Y',strtotime($model->dob)));
});
}
Profile Controller
public function store(Request $request)
{
$profile = new Profile();
$profile->weight = $request->get('weight');
$profile->height = $request->get('height');
$profile->dob = $request->get('dob');
$profile->age;
$profile->bmi;
$profile->save();
return back()->with('success', 'Your profile has been updated.');
}
But im receiving an error
Illuminate \ Database \ QueryException (42S22)
SQLSTATE[42S22]: Column not found: 1054 Unknown column 'weight' in 'field list' (SQL: insert into `users` (`weight`, `height`, `dob`, `bmi`, `age`, `created_by`, `updated_by`, `updated_at`, `created_at`) values (60, 175, 1988-04-03, 0.0019591836734694, 30, 2, 2, 2018-03-08 20:06:02, 2018-03-08 20:06:02))

Answer 1

You could do this in the boot method of the model:
protected static function boot() {
parent::boot();
static::saving(function($model){
$model->bmi = $model->weight / ($model->height * $model->height);
});
}

Answer 2

What is the best practice to store calculated fields in database.
In general, don't. It's redundant - your database already has all the information needed to compute it, so why store redundant data?
Instead, put an accessor on the model:
public function getBmiAttribute() {
return ($this->weight / ($this->height * $this->height));
}
You can then do $this->bmi to get the computed value.
If you must have it in the database, use an Eloquent event. You'd probably want to hook into the saving event.

Answer 3

As of Laravel 5.3 & MySQL 5.7, you can use Column Modifiers virtualAs and storedAs to create a VIRTUAL (evaluated when rows are read) or STORED (evaluated and stored when rows are inserted or updated) generated column for MySQL.
I've made use of virtualAs in the example below...
Schema::create('results', function (Blueprint $table) {
$table->bigIncrements('id');
...
$table->time('predicted_time')->nullable()->default(NULL);
$table->time('handicap')->nullable()->default(NULL);
$table->time('finish_time')->nullable()->default(NULL);
$table->time('actual_time')->virtualAs('TIMEDIFF(finish_time, handicap)');
$table->time('offset')->virtualAs('TIMEDIFF(actual_time, predicted_time)');
...
});

Answer 4

What is the best practice to store calculated fields in database.
It depends on your use case. If you're using a relational database, and your use case does not involve big data (in terms of volume, variety or velocity), the best practice is to not store calculated fields and calculate them on the fly.
If you're using a noSQL database (such as MongoDB, which is supported by Laravel) or Hadoop, the best practice is to store calculated fields to avoid computational time.
In A Nutshell
It's a tradeoff between time complexity and space/storage complexity.
For big data / noSQL systems, store calculated fields especially if
they are computationally complex. For a relational database, calculate
them on the fly and keep your data non-redundant
Laravel Solution for RDBMS:
Use accessors like so:
public function getBmiAttribute($value)
{
return ($this->weight / ($this->height * $this->height));
}
Laravel Solution for NoSql
Use model events like so:
protected static function boot() {
parent::boot();
static::saving(function($model){
$model->bmi = $model->weight / ($model->height * $model->height);
});
}

Answer 5

If your DBMS supports computed columns (aka generated columns), you might consider utilizing them.
Highlights:
Computed columns don't persist the data to the database (or if they do technically they'll be managed by the DBMS), so you're not duplicating any data or making anything redundant
The calculation is then available outside of the application's code base for anything to use. For example, if there is a need to develop reporting with raw SQL queries then the calculation will be available there.
Eloquent (Laravel's default ORM) will pick up the column without you necessarily needing to define it anywhere.
You could execute code to create it during a migration. For example, I had a use case where I wanted to simplify determining if something was currently published and I did it like this:
Schema::table('article', function (Blueprint $table) {
$table->dateTime('published_at')->nullable();
$table->dateTime('unpublished_at')->nullable();
});
$sql = "ALTER TABLE article ADD is_published AS CAST(CASE WHEN GETUTCDATE() BETWEEN ISNULL(published_at, '2050-01-01') AND ISNULL(unpublished_at, '2050-01-01') THEN 1 ELSE 0 END AS BIT);";
DB::connection()->getPdo()->exec($sql);
Then after retrieving a record from the database I can determine if it's published by simply checking...
if ($article->is_published) ...
And if I want to query it from the database I can easily do...
SELECT * FROM article WHERE is_published = 1
Note, this syntax is for T-SQL since the RDBMS for my project is MS SQL Server. However, other popular DBMS have similar features.
MS SQL Server: Computed Columns
ALTER TABLE dbo.Products ADD RetailValue AS (QtyAvailable * UnitPrice * 1.35);
MySQL: Generated Columns
ALTER TABLE t1 ADD COLUMN c2 INT GENERATED ALWAYS AS (c1 + 1) STORED;
Oracle: Virtual Columns
ALTER TABLE emp2 ADD (income AS (salary + (salary*commission_pct)));
PostgreSQL doesn't seem to support it (yet), but this SO answer provides a decent workaround.

Answer 6

another way to make this
making a computed column in the migration
this working with all DB:-
$table->computed ('tax','price * 0.27');
but this working with MySQL DB:-
$table->integer('tax')->virtualAs('price * 0.27')->nullable();
$table->integer('discount')->storedAs('price - 100')->nullable();