Associations: Linking Models Together¶

Relationship Types¶

The four association types in CakePHP are: hasOne, hasMany, belongsTo, and hasAndBelongsToMany (HABTM).

To further clarify which way around the associations are defined in the models: If the table of the model contains the foreign key (other_model_id), the relation type in this model is always a Model belongsTo OtherModel relation!

Associations are defined by creating a class variable named after the association you are defining. The class variable can sometimes be as simple as a string, but can be as complex as a multidimensional array used to define association specifics.

class User extends AppModel {
    public $hasOne = 'Profile';
    public $hasMany = array(
        'Recipe' => array(
            'className' => 'Recipe',
            'conditions' => array('Recipe.approved' => '1'),
            'order' => 'Recipe.created DESC'
        )
    );
}

In the above example, the first instance of the word ‘Recipe’ is what is termed an ‘Alias’. This is an identifier for the relationship, and can be anything you choose. Usually, you will choose the same name as the class that it references. However, aliases for each model must be unique across the app. For example, it is appropriate to have:

class User extends AppModel {
    public $hasMany = array(
        'MyRecipe' => array(
            'className' => 'Recipe',
        )
    );
    public $hasAndBelongsToMany = array(
        'MemberOf' => array(
            'className' => 'Group',
        )
    );
}

class Group extends AppModel {
    public $hasMany = array(
        'MyRecipe' => array(
            'className' => 'Recipe',
        )
    );
    public $hasAndBelongsToMany = array(
        'Member' => array(
            'className' => 'User',
        )
    );
}

but the following will not work well in all circumstances:

class User extends AppModel {
    public $hasMany = array(
        'MyRecipe' => array(
            'className' => 'Recipe',
        )
    );
    public $hasAndBelongsToMany = array(
        'Member' => array(
            'className' => 'Group',
        )
    );
}

class Group extends AppModel {
    public $hasMany = array(
        'MyRecipe' => array(
            'className' => 'Recipe',
        )
    );
    public $hasAndBelongsToMany = array(
        'Member' => array(
            'className' => 'User',
        )
    );
}

because here we have the alias ‘Member’ referring to both the User (in Group) and the Group (in User) model in the HABTM associations. Choosing non-unique names for model aliases across models can cause unexpected behavior.

CakePHP will automatically create links between associated model objects. So for example in your User model you can access the Recipe model as:

$this->Recipe->someFunction();

Similarly in your controller you can access an associated model simply by following your model associations:

$this->User->Recipe->someFunction();

hasOne¶

Let’s set up a User model with a hasOne relationship to a Profile model.

First, your database tables need to be keyed correctly. For a hasOne relationship to work, one table has to contain a foreign key that points to a record in the other. In this case, the profiles table will contain a field called user_id. The basic pattern is:

hasOne: the other model contains the foreign key.

The User model file will be saved in /app/Model/User.php. To define the ‘User hasOne Profile’ association, add the $hasOne property to the model class. Remember to have a Profile model in /app/Model/Profile.php, or the association won’t work:

class User extends AppModel {
    public $hasOne = 'Profile';
}

There are two ways to describe this relationship in your model files. The simplest method is to set the $hasOne attribute to a string containing the class name of the associated model, as we’ve done above.

If you need more control, you can define your associations using array syntax. For example, you might want to limit the association to include only certain records.

class User extends AppModel {
    public $hasOne = array(
        'Profile' => array(
            'className' => 'Profile',
            'conditions' => array('Profile.published' => '1'),
            'dependent' => true
        )
    );
}

Possible keys for hasOne association arrays include:

• className: the class name of the model being associated to the current model. If you’re defining a ‘User hasOne Profile’ relationship, the className key should equal ‘Profile’.

• foreignKey: the name of the foreign key found in the other model. This is especially handy if you need to define multiple hasOne relationships. The default value for this key is the underscored, singular name of the current model, suffixed with ‘_id’. In the example above, it would default to ‘user_id’. If the other model uses another name than ‘id’ for the primary key, then make sure to specify it using the $primaryKey property of your Model class (in the example above, in the ‘Profile’ Model class). Otherwise cascade deletes will not work.

• conditions: an array of find()-compatible conditions or SQL strings such as array(‘Profile.approved’ => true)

• fields: A list of fields to be retrieved when the associated model data is fetched. Returns all fields by default.

• order: an array of find()-compatible order clauses or SQL strings such as array(‘Profile.last_name’ => ‘ASC’)

• dependent: When the dependent key is set to true, and the model’s delete() method is called with the cascade parameter set to true, associated model records are also deleted. In this case, we set it true so that deleting a User will also delete her associated Profile.

Once this association has been defined, find operations on the User model will also fetch a related Profile record if it exists:

//Sample results from a $this->User->find() call.

Array
(
    [User] => Array
        (
            [id] => 121
            [name] => Gwoo the Kungwoo
            [created] => 2007-05-01 10:31:01
        )
    [Profile] => Array
        (
            [id] => 12
            [user_id] => 121
            [skill] => Baking Cakes
            [created] => 2007-05-01 10:31:01
        )
)

belongsTo¶

Now that we have Profile data access from the User model, let’s define a belongsTo association in the Profile model in order to get access to related User data. The belongsTo association is a natural complement to the hasOne and hasMany associations: it allows us to see the data from the other direction.

When keying your database tables for a belongsTo relationship, follow this convention:

belongsTo: the current model contains the foreign key.

We can define the belongsTo association in our Profile model at /app/Model/Profile.php using the string syntax as follows:

class Profile extends AppModel {
    public $belongsTo = 'User';
}

We can also define a more specific relationship using array syntax:

class Profile extends AppModel {
    public $belongsTo = array(
        'User' => array(
            'className' => 'User',
            'foreignKey' => 'user_id'
        )
    );
}

Possible keys for belongsTo association arrays include:

• className: the class name of the model being associated to the current model. If you’re defining a ‘Profile belongsTo User’ relationship, the className key should equal ‘User’.

• foreignKey: the name of the foreign key found in the current model. This is especially handy if you need to define multiple belongsTo relationships. The default value for this key is the underscored, singular name of the other model, suffixed with _id.

• conditions: an array of find() compatible conditions or SQL strings such as array('User.active' => true)

• type: the type of the join to use in the SQL query. The default is ‘LEFT’, which may not fit your needs in all situations. The value ‘INNER’ may be helpful (when used with some conditions) when you want everything from your main and associated models or nothing at all.

• fields: A list of fields to be retrieved when the associated model data is fetched. Returns all fields by default.

• order: an array of find() compatible order clauses or SQL strings such as array('User.username' => 'ASC')

• counterCache: If set to true, the associated Model will automatically increase or decrease the “[singular_model_name]_count” field in the foreign table whenever you do a save() or delete(). If it’s a string, then it’s the field name to use. The value in the counter field represents the number of related rows. You can also specify multiple counter caches by defining an array. See Multiple counterCache.

• counterScope: Optional conditions array to use for updating counter cache field.

Once this association has been defined, find operations on the Profile model will also fetch a related User record if it exists:

//Sample results from a $this->Profile->find() call.

Array
(
   [Profile] => Array
        (
            [id] => 12
            [user_id] => 121
            [skill] => Baking Cakes
            [created] => 2007-05-01 10:31:01
        )
    [User] => Array
        (
            [id] => 121
            [name] => Gwoo the Kungwoo
            [created] => 2007-05-01 10:31:01
        )
)

my_model_count

class ImageComment extends AppModel {
    public $belongsTo = array(
        'Image' => array(
            'counterCache' => true,
        )
    );
}

class ImageComment extends AppModel {
    public $belongsTo = array(
        'Image' => array(
            'counterCache' => 'active_comment_count', //custom field name
            // only count if "ImageComment" is active = 1
            'counterScope' => array(
              'ImageComment.active' => 1
            )
        )
    );
}

class Message extends AppModel {
    public $belongsTo = array(
        'User' => array(
            'counterCache' => array(
                'messages_read' => array('Message.is_read' => 1),
                'messages_unread' => array('Message.is_read' => 0)
            )
        )
    );
}

hasMany¶

Next step: defining a “User hasMany Comment” association. A hasMany association will allow us to fetch a user’s comments when we fetch a User record.

When keying your database tables for a hasMany relationship, follow this convention:

hasMany: the other model contains the foreign key.

We can define the hasMany association in our User model at /app/Model/User.php using the string syntax as follows:

class User extends AppModel {
    public $hasMany = 'Comment';
}

We can also define a more specific relationship using array syntax:

class User extends AppModel {
    public $hasMany = array(
        'Comment' => array(
            'className' => 'Comment',
            'foreignKey' => 'user_id',
            'conditions' => array('Comment.status' => '1'),
            'order' => 'Comment.created DESC',
            'limit' => '5',
            'dependent' => true
        )
    );
}

Possible keys for hasMany association arrays include:

• className: the class name of the model being associated to the current model. If you’re defining a ‘User hasMany Comment’ relationship, the className key should equal ‘Comment.’

• foreignKey: the name of the foreign key found in the other model. This is especially handy if you need to define multiple hasMany relationships. The default value for this key is the underscored, singular name of the actual model, suffixed with ‘_id’.

• conditions: an array of find() compatible conditions or SQL strings such as array(‘Comment.visible’ => true)

• order: an array of find() compatible order clauses or SQL strings such as array(‘Profile.last_name’ => ‘ASC’)

• limit: The maximum number of associated rows you want returned.

• offset: The number of associated rows to skip over (given the current conditions and order) before fetching and associating.

• dependent: When dependent is set to true, recursive model deletion is possible. In this example, Comment records will be deleted when their associated User record has been deleted.

• exclusive: When exclusive is set to true, recursive model deletion does the delete with a deleteAll() call, instead of deleting each entity separately. This greatly improves performance, but may not be ideal for all circumstances.

• finderQuery: A complete SQL query CakePHP can use to fetch associated model records. This should be used in situations that require highly customized results. If a query you’re building requires a reference to the associated model ID, use the special {$__cakeID__$} marker in the query. For example, if your Apple model hasMany Orange, the query should look something like this: SELECT Orange.* from oranges as Orange WHERE Orange.apple_id = {$__cakeID__$};

Once this association has been defined, find operations on the User model will also fetch related Comment records if they exist:

//Sample results from a $this->User->find() call.

Array
(
    [User] => Array
        (
            [id] => 121
            [name] => Gwoo the Kungwoo
            [created] => 2007-05-01 10:31:01
        )
    [Comment] => Array
        (
            [0] => Array
                (
                    [id] => 123
                    [user_id] => 121
                    [title] => On Gwoo the Kungwoo
                    [body] => The Kungwooness is not so Gwooish
                    [created] => 2006-05-01 10:31:01
                )
            [1] => Array
                (
                    [id] => 124
                    [user_id] => 121
                    [title] => More on Gwoo
                    [body] => But what of the 'Nut?
                    [created] => 2006-05-01 10:41:01
                )
        )
)

One thing to remember is that you’ll need a complementary Comment belongsTo User association in order to get the data from both directions. What we’ve outlined in this section empowers you to get Comment data from the User. Adding the Comment belongsTo User association in the Comment model enables you to get User data from the Comment model, completing the connection and allowing the flow of information from either model’s perspective.

hasAndBelongsToMany (HABTM)¶

All right. At this point, you can already call yourself a CakePHP model associations professional. You’re already well versed in the three associations that take up the bulk of object relations.

Let’s tackle the final relationship type: hasAndBelongsToMany, or HABTM. This association is used when you have two models that need to be joined up, repeatedly, many times, in many different ways.

The main difference between hasMany and HABTM is that a link between models in HABTM is not exclusive. For example, we’re about to join up our Recipe model with an Ingredient model using HABTM. Using tomatoes as an Ingredient for my grandma’s spaghetti recipe doesn’t “use up” the ingredient. I can also use it for a salad Recipe.

Links between hasMany associated objects are exclusive. If my User hasMany Comments, a comment is only linked to a specific user. It’s not up for grabs.

Moving on. We’ll need to set up an extra table in the database to handle HABTM associations. This new join table’s name needs to include the names of both models involved, in alphabetical order, and separated with an underscore ( _ ). The contents of the table should be two fields that are foreign keys (which should be integers) pointing to the primary keys of the involved models. To avoid any issues, don’t define a combined primary key for these two fields. If your application requires a unique index, you can define one. If you plan to add any extra information to this table, or use a ‘with’ model, you should add an additional primary key field (by convention ‘id’).

HABTM requires a separate join table that includes both model names.

Make sure primary keys in tables cakes and recipes have “id” fields as assumed by convention. If they’re different than assumed, they must be changed in model’s primaryKey.

Once this new table has been created, we can define the HABTM association in the model files. We’re going to skip straight to the array syntax this time:

class Recipe extends AppModel {
    public $hasAndBelongsToMany = array(
        'Ingredient' =>
            array(
                'className' => 'Ingredient',
                'joinTable' => 'ingredients_recipes',
                'foreignKey' => 'recipe_id',
                'associationForeignKey' => 'ingredient_id',
                'unique' => true,
                'conditions' => '',
                'fields' => '',
                'order' => '',
                'limit' => '',
                'offset' => '',
                'finderQuery' => '',
                'with' => ''
            )
    );
}

Possible keys for HABTM association arrays include:

• className: the class name of the model being associated to the current model. If you’re defining a ‘Recipe HABTM Ingredient’ relationship, the className key should equal ‘Ingredient’.

• joinTable: The name of the join table used in this association (if the current table doesn’t adhere to the naming convention for HABTM join tables).

• with: Defines the name of the model for the join table. By default CakePHP will auto-create a model for you. Using the example above it would be called IngredientsRecipe. By using this key you can override this default name. The join table model can be used just like any “regular” model to access the join table directly. By creating a model class with such name and filename, you can add any custom behavior to the join table searches, such as adding more information/columns to it.

• foreignKey: the name of the foreign key found in the current model. This is especially handy if you need to define multiple HABTM relationships. The default value for this key is the underscored, singular name of the current model, suffixed with ‘_id’.

• associationForeignKey: the name of the foreign key found in the other model. This is especially handy if you need to define multiple HABTM relationships. The default value for this key is the underscored, singular name of the other model, suffixed with ‘_id’.

• unique: boolean or string keepExisting.

  • If true (default value) CakePHP will first delete existing relationship records in the foreign keys table before inserting new ones. Existing associations need to be passed again when updating.

  • When false, CakePHP will insert the specified new relationship records and leave any existing relationship records in place, possibly resulting in duplicate relationship records.

  • When set to keepExisting, the behavior is similar to true, but with an additional check so that if any of the records to be added are duplicates of an existing relationship record, the existing relationship record is not deleted, and the duplicate is ignored. This can be useful if, for example, the join table has additional data in it that needs to be retained.

• conditions: an array of find()-compatible conditions or SQL string. If you have conditions on an associated table, you should use a ‘with’ model, and define the necessary belongsTo associations on it.

• fields: A list of fields to be retrieved when the associated model data is fetched. Returns all fields by default.

• order: an array of find()-compatible order clauses or SQL strings

• limit: The maximum number of associated rows you want returned.

• offset: The number of associated rows to skip over (given the current conditions and order) before fetching and associating.

• finderQuery: A complete SQL query CakePHP can use to fetch associated model records. This should be used in situations that require highly customized results.

Once this association has been defined, find operations on the Recipe model will also fetch related Ingredient records if they exist:

// Sample results from a $this->Recipe->find() call.

Array
(
    [Recipe] => Array
        (
            [id] => 2745
            [name] => Chocolate Frosted Sugar Bombs
            [created] => 2007-05-01 10:31:01
            [user_id] => 2346
        )
    [Ingredient] => Array
        (
            [0] => Array
                (
                    [id] => 123
                    [name] => Chocolate
                )
           [1] => Array
                (
                    [id] => 124
                    [name] => Sugar
                )
           [2] => Array
                (
                    [id] => 125
                    [name] => Bombs
                )
        )
)

Remember to define a HABTM association in the Ingredient model if you’d like to fetch Recipe data when using the Ingredient model.

hasMany through (The Join Model)¶

It is sometimes desirable to store additional data with a many-to-many association. Consider the following

Student hasAndBelongsToMany Course

Course hasAndBelongsToMany Student

In other words, a Student can take many Courses and a Course can be taken by many Students. This is a simple many-to-many association demanding a table such as this:

id | student_id | course_id

Now what if we want to store the number of days that were attended by the student on the course and their final grade? The table we’d want would be:

id | student_id | course_id | days_attended | grade

The trouble is, hasAndBelongsToMany will not support this type of scenario because when hasAndBelongsToMany associations are saved, the association is deleted first. You would lose the extra data in the columns as it is not replaced in the new insert.

The way to implement our requirement is to use a join model, otherwise known as a hasMany through association. That is, the association is a model itself. So, we can create a new model CourseMembership. Take a look at the following models.

// Student.php
class Student extends AppModel {
    public $hasMany = array(
        'CourseMembership'
    );
}

// Course.php

class Course extends AppModel {
    public $hasMany = array(
        'CourseMembership'
    );
}

// CourseMembership.php

class CourseMembership extends AppModel {
    public $belongsTo = array(
        'Student', 'Course'
    );
}

The CourseMembership join model uniquely identifies a given Student’s participation on a Course in addition to extra meta-information.

Join models are pretty useful things to be able to use, and CakePHP makes it easy to do so with its built-in hasMany and belongsTo associations and saveAll feature.

Creating and Destroying Associations on the Fly¶

Sometimes it becomes necessary to create and destroy model associations on the fly. This may be for any number of reasons:

• You want to reduce the amount of associated data fetched, but all your associations are on the first level of recursion.

• You want to change the way an association is defined in order to sort or filter associated data.

This association creation and destruction is done using the CakePHP model bindModel() and unbindModel() methods. (There is also a very helpful behavior called “Containable”. Please refer to the manual section about Built-in behaviors for more information.) Let’s set up a few models so we can see how bindModel() and unbindModel() work. We’ll start with two models:

class Leader extends AppModel {
    public $hasMany = array(
        'Follower' => array(
            'className' => 'Follower',
            'order' => 'Follower.rank'
        )
    );
}

class Follower extends AppModel {
    public $name = 'Follower';
}

Now, in the LeadersController, we can use the find() method in the Leader model to fetch a Leader and its associated followers. As you can see above, the association array in the Leader model defines a “Leader hasMany Followers” relationship. For demonstration purposes, let’s use unbindModel() to remove that association in a controller action:

public function some_action() {
    // This fetches Leaders, and their associated Followers
    $this->Leader->find('all');

    // Let's remove the hasMany...
    $this->Leader->unbindModel(
        array('hasMany' => array('Follower'))
    );

    // Now using a find function will return
    // Leaders, with no Followers
    $this->Leader->find('all');

    // NOTE: unbindModel only affects the very next
    // find function. An additional find call will use
    // the configured association information.

    // We've already used find('all') after unbindModel(),
    // so this will fetch Leaders with associated
    // Followers once again...
    $this->Leader->find('all');
}

Here’s the basic usage pattern for unbindModel():

$this->Model->unbindModel(
    array('associationType' => array('associatedModelClassName'))
);

Now that we’ve successfully removed an association on the fly, let’s add one. Our as-of-yet unprincipled Leader needs some associated Principles. The model file for our Principle model is bare, except for the public $name statement. Let’s associate some Principles to our Leader on the fly (but remember, only for the following find operation). This function appears in the LeadersController:

public function another_action() {
    // There is no Leader hasMany Principles in
    // the leader.php model file, so a find here
    // only fetches Leaders.
    $this->Leader->find('all');

    // Let's use bindModel() to add a new association
    // to the Leader model:
    $this->Leader->bindModel(
        array('hasMany' => array(
                'Principle' => array(
                    'className' => 'Principle'
                )
            )
        )
    );

    // If we need keep this association after model reset
    // we will pass a second boolean parameter like this:
    $this->Leader->bindModel(
        array('hasMany' => array(
                'Principle' => array(
                    'className' => 'Principle'
                )
            )
        ),
        false
    );

    // Now that we're associated correctly,
    // we can use a single find function to fetch
    // Leaders with their associated principles:
    $this->Leader->find('all');
}

There you have it. The basic usage for bindModel() is the encapsulation of a normal association array inside an array whose key is named after the type of association you are trying to create:

$this->Model->bindModel(
    array('associationName' => array(
            'associatedModelClassName' => array(
                // normal association keys go here...
            )
        )
    )
);

Even though the newly bound model doesn’t need any sort of association definition in its model file, it will still need to be correctly keyed in order for the new association to work properly.

Multiple relations to the same model¶

There are cases where a Model has more than one relation to another Model. For example, you might have a Message model that has two relations to the User model: one relation to the user who sends a message, and a second to the user who receives the message. The messages table will have a field user_id, but also a field recipient_id. Now your Message model can look something like:

class Message extends AppModel {
    public $belongsTo = array(
        'Sender' => array(
            'className' => 'User',
            'foreignKey' => 'user_id'
        ),
        'Recipient' => array(
            'className' => 'User',
            'foreignKey' => 'recipient_id'
        )
    );
}

Recipient is an alias for the User model. Now let’s see what the User model would look like:

class User extends AppModel {
    public $hasMany = array(
        'MessageSent' => array(
            'className' => 'Message',
            'foreignKey' => 'user_id'
        ),
        'MessageReceived' => array(
            'className' => 'Message',
            'foreignKey' => 'recipient_id'
        )
    );
}

It is also possible to create self associations as shown below:

class Post extends AppModel {

    public $belongsTo = array(
        'Parent' => array(
            'className' => 'Post',
            'foreignKey' => 'parent_id'
        )
    );

    public $hasMany = array(
        'Children' => array(
            'className' => 'Post',
            'foreignKey' => 'parent_id'
        )
    );
}

Fetching a nested array of associated records:

If your table has a parent_id field, you can also use find(‘threaded’) to fetch a nested array of records using a single query without setting up any associations.

Joining tables¶

In SQL, you can combine related tables using the JOIN statement. This allows you to perform complex searches across multiple tables (for example, search posts given several tags).

In CakePHP, some associations (belongsTo and hasOne) perform automatic joins to retrieve data, so you can issue queries to retrieve models based on data in the related one.

But this is not the case with hasMany and hasAndBelongsToMany associations. Here is where forcing joins comes to the rescue. You only have to define the necessary joins to combine tables and get the desired results for your query.

To force a join between tables, you need to use the “modern” syntax for Model::find(), adding a ‘joins’ key to the $options array. For example:

$options['joins'] = array(
    array('table' => 'channels',
        'alias' => 'Channel',
        'type' => 'LEFT',
        'conditions' => array(
            'Channel.id = Item.channel_id',
        )
    )
);

$Item->find('all', $options);

In the above example, a model called Item is left-joined to the channels table. You can alias the table with the Model name, so the retrieved data complies with the CakePHP data structure.

The keys that define the join are the following:

• table: The table for the join.

• alias: An alias to the table. The name of the model associated with the table is the best bet.

• type: The type of join: inner, left or right.

• conditions: The conditions to perform the join.

With joins, you could add conditions based on Related model fields:

$options['joins'] = array(
    array('table' => 'channels',
        'alias' => 'Channel',
        'type' => 'LEFT',
        'conditions' => array(
            'Channel.id = Item.channel_id',
        )
    )
);

$options['conditions'] = array(
    'Channel.private' => 1
);

$privateItems = $Item->find('all', $options);

You could perform several joins as needed in hasAndBelongsToMany:

Suppose there is a Book hasAndBelongsToMany Tag association. This relation uses a books_tags table as a join table, so you need to join the books table to the books_tags table, and this with the tags table:

$options['joins'] = array(
    array('table' => 'books_tags',
        'alias' => 'BooksTag',
        'type' => 'inner',
        'conditions' => array(
            'Book.id = BooksTag.book_id'
        )
    ),
    array('table' => 'tags',
        'alias' => 'Tag',
        'type' => 'inner',
        'conditions' => array(
            'BooksTag.tag_id = Tag.id'
        )
    )
);

$options['conditions'] = array(
    'Tag.tag' => 'Novel'
);

$books = $Book->find('all', $options);

Using joins allows you to have maximum flexibility in how CakePHP handles associations and fetches the data. However, in most cases, you can use other tools to achieve the same results such as correctly defining associations, binding models on the fly and using the Containable behavior. This feature should be used with care because it could lead, in a few cases, into ill-formed SQL queries if combined with any of the former techniques described for associating models.