The answers above were not sufficient for me to understand what was going on, so after delving into it more I think I have a way of explaining it that will make sense for people who struggled like I did to understand.
inversedBy and mappedBy are used by the INTERNAL DOCTRINE engine to reduce the number of SQL queries it has to do to get the information you need. To be clear if you don't add inversedBy or mappedBy your code will still work but will not be optimized.
So for example, look at the classes below:
class Task
{
/**
* @var int
*
* @ORM\Column(name="id", type="integer")
* @ORM\Id
* @ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* @var string
*
* @ORM\Column(name="task", type="string", length=255)
*/
private $task;
/**
* @var \DateTime
*
* @ORM\Column(name="dueDate", type="datetime")
*/
private $dueDate;
/**
* @ORM\ManyToOne(targetEntity="Category", inversedBy="tasks", cascade={"persist"})
* @ORM\JoinColumn(name="category_id", referencedColumnName="id")
*/
protected $category;
}
class Category
{
/**
* @var int
*
* @ORM\Column(name="id", type="integer")
* @ORM\Id
* @ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* @var string
*
* @ORM\Column(name="name", type="string", length=255)
*/
private $name;
/**
* @ORM\OneToMany(targetEntity="Task", mappedBy="category")
*/
protected $tasks;
}
These classes if you were to run the command to generate the schema (for example, bin/console doctrine:schema:update --force --dump-sql) you will notice that the Category table does not have a column on it for tasks. (this is because it does not have a column annotation on it)
The important thing to understand here is that the variable tasks is only there so the internal doctrine engine can use the reference above it which says its mappedBy Category. Now... don't be confused here like I was... Category is NOT referring TO THE CLASS NAME, its referring to the property on the Task class called 'protected $category'.
Like wise, on the Tasks class the property $category mentions it is inversedBy="tasks", notice this is plural, this is NOT THE PLURAL OF THE CLASS NAME, but just because the property is called 'protected $tasks' in the Category class.
Once you understand this it becomes very easy to understand what inversedBy and mappedBy are doing and how to use them in this situation.
The side that is referencing the foreign key like 'tasks' in my example always gets the inversedBy attribute because it needs to know what class (via the targetEntity command) and what variable (inversedBy=) on that class to 'work backwards' so to speak and get the category information from. An easy way to remember this, is the class that would have the foreignkey_id is the one that needs to have inversedBy.
Where as with category, and its $tasks property (which is not on the table remember, just only part of the class for optimization purposes) is MappedBy 'tasks', this creates the relationship officially between the two entities so that doctrine can now safely use JOIN SQL statements instead of two separate SELECT statements. Without mappedBy, the doctrine engine would not know from the JOIN statement it will create what variable in the class 'Task' to put the category information.
Hope this explains it a bit better.
