doctrine2/manual/en/best-practices.txt

> **NOTE**
> The best practices mentioned here that affect database design generally refer to best
> practices when working with Doctrine and do not necessarily reflect best practices for
> database design in general.

## Don't use public properties on entities

It is very important that you don't map public properties on entities, but only protected or private ones.
The reason for this is simple, whenever you access a public property of a proxy object that hasn't been initialized
yet the return value will be null. Doctrine cannot hook into this process and magically make the entity lazy load.

This can create situations where it is very hard to debug the current failure. We therefore urge you to map only
private and protected properties on entities and use getter methods or magic __get() to access them.

## Constrain relationships as much as possible

It is important to constrain relationships as much as possible. This means:

* Impose a traversal direction (avoid bidirectional associations if possible)
* Eliminate nonessential associations

This has several benefits:

* Reduced coupling in your domain model
* Simpler code in your domain model (no need to maintain bidirectionality properly)
* Less work for Doctrine

## Avoid composite keys

Even though Doctrine fully supports composite keys it is best not to use them if possible. Composite keys require additional work by Doctrine and thus have a higher probability of errors.

## Use events judiciously

The event system of Doctrine is great and fast. Even though making heavy use of events, especially lifecycle events, can have a negative impact on the performance of your application. Thus you should use events judiciously.

## Use cascades judiciously

Automatic cascades of the persist/remove/merge/etc. operations are very handy but should be used wisely. Do NOT simply add all cascades to all associations. Think about which cascades actually do make sense for you for a particular association, given the scenarios it is most likely used in.

## Don't use special characters

Avoid using any non-ASCII characters in class, field, table or column names. Doctrine itself is not unicode-safe in many places and will not be until PHP itself is fully unicode-aware (PHP6).

## Don't use identifier quoting

Identifier quoting is a workaround for using reserved words that often causes problems in edge cases. Do not use identifier quoting and avoid using reserved words as table or column names.

## Initialize collections in the constructor

It is recommended best practice to initialize any business collections in entities in the constructor. Example:

    <?php
    namespace MyProject\Model;
    use Doctrine\Common\Collections\ArrayCollection;
    
    class User {
        private $addresses;
        private $articles;
        
        public function __construct() {
            $this->addresses = new ArrayCollection;
            $this->articles = new ArrayCollection;
        }
    }

## Don't map foreign keys to fields in an entity

Foreign keys have no meaning whatsoever in an object model. Foreign keys are how a relational database establishes relationships. Your object model establishes relationships through object references. Thus mapping foreign keys to object fields heavily leaks details of the relational model into the object model, something you really should not do.

## Use explicit transaction demarcation

While Doctrine will automatically wrap all DML operations in a transaction on flush(), it is considered best practice to explicitly set the transaction boundaries yourself.
Otherwise every single query is wrapped in a small transaction (Yes, SELECT queries, too) since you can not talk to your database outside of a transaction.
While such short transactions for read-only (SELECT) queries generally don't have any noticeable performance impact, it is still preferable to use fewer, well-defined transactions
that are established through explicit transaction boundaries.