TinyORM/docs/tinyorm-relationships.md

TinyORM: Relationships

• Introduction
• Defining Relationships
  • One To One
  • One To Many
  • One To Many (Inverse) / Belongs To
• Many To Many Relationships coming soon
• Querying Relations
  • Relationship Methods
• Eager Loading
  • Lazy Eager Loading
• Inserting & Updating Related Models
• Touching Parent Timestamps

Introduction

Database tables are often related to one another. For example, a blog post may have many comments or an order could be related to the user who placed it. TinyORM makes managing and working with these relationships easy, and supports basic relationships:

• One To One
• One To Many
• Many To Many coming soon

Defining Relationships

TinyORM relationships are defined as methods on your TinyORM model classes. Since relationships also serve as powerful query builders, defining relationships as methods provides powerful method chaining and querying capabilities. For example, we may chain additional query constraints on this posts relationship:

user->posts()->whereEq("active", 1).get();

But, before diving too deep into using relationships, let's learn how to define each type of relationship supported by TinyORM.

One To One

A one-to-one relationship is a very basic type of database relationship. For example, a User model might be associated with one Phone model. To define this relationship, we will place a phone method on the User model. The phone method should call the hasOne method and return its result. The hasOne<Related> method is available to your model via the model's Orm::Tiny::BaseModel<Model, AllRelations...> base class:

#ifndef USER_H
#define USER_H

#include <orm/tiny/basemodel.hpp>

#include "models/phone.hpp"

using Orm::Tiny::BaseModel;
using Orm::Tiny::Relations::Relation;

class User final : public BaseModel<User, Phone>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get the phone associated with the user. */
    std::unique_ptr<Relation<User, Phone>>
    phone()
    {
        return hasOne<Phone>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "phone")
            relationVisited<Phone>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"phone", &User::phone},
    };
};

#endif // USER_H

The Related template argument provided to the hasOne<Related> method is the type-id of the related model class. Once the relationship is defined, we may retrieve the related record using BaseModel's getRelationValue<Related, Tag> method:

auto phone = User::find(1)->getRelationValue<Phone, Orm::One>("phone");

TinyORM determines the foreign key of the relationship based on the parent model name. In this case, the Phone model is automatically assumed to have a user_id foreign key. If you wish to override this convention, you may pass a first argument to the hasOne method:

return hasOne<Phone>("foreign_key");

Additionally, TinyORM assumes that the foreign key should have a value matching the primary key column of the parent. In other words, TinyORM will look for the value of the user's id column in the user_id column of the Phone record. If you would like the relationship to use a primary key value other than id or your model's u_primaryKey data member, you may pass a second argument to the hasOne method:

return hasOne<Phone>("foreign_key", "local_key");

Defining The Inverse Of The Relationship

So, we can access the Phone model from our User model. Next, let's define a relationship on the Phone model that will let us access the user that owns the phone. We can define the inverse of a hasOne relationship using the belongsTo<Related> method:

#ifndef PHONE_H
#define PHONE_H

#include <orm/tiny/basemodel.hpp>

#include "models/user.hpp"

using Orm::Tiny::BaseModel;
using Orm::Tiny::Relations::Relation;

class Phone final : public BaseModel<Phone, User>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get the user that owns the phone. */
    std::unique_ptr<Relation<Phone, User>>
    user()
    {
        return belongsTo<User>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "user")
            relationVisited<User>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"user", &Phone::user},
    };
};

#endif // PHONE_H

When invoking the user method, TinyORM will attempt to find a User model that has an id which matches the user_id column on the Phone model.

TinyORM determines the foreign key name by examining the type-name of the Related template parameter and suffixing the type-name with _id. So, in this case, TinyORM assumes that the Phone model has a user_id column.

However, if the foreign key on the Phone model is not user_id, you may pass a custom key name as the first argument to the belongsTo method:

using Orm::Tiny::Relations::Relation;

/*! Get the user that owns the phone. */
std::unique_ptr<Relation<Phone, User>>
user()
{
    return belongsTo<User>("foreign_key");
}

If the parent model does not use id as its primary key, or you wish to find the associated model using a different column, you may pass a second argument to the belongsTo method specifying the parent table's custom key:

/*! Get the user that owns the phone. */
std::unique_ptr<Relation<Phone, User>>
user()
{
    return belongsTo<User>("foreign_key", "owner_key");
}

The third belongsTo parameter is the relation name, if you pass it, the foreign key name will be determined from it. By convention, TinyORM will "snake case" this relation name and suffix it with a _ followed by the name of the parent model's primary key column, the __func__ predefined identifier is ideal for this:

/*! Get the user that owns the phone. */
std::unique_ptr<Relation<Phone, User>>
someUser()
{
    return belongsTo<User>({}, {}, __func__); // the foreign key will be some_user_id
}

One To Many

A one-to-many relationship is used to define relationships where a single model is the parent to one or more child models. For example, a blog post may have an infinite number of comments. Like all other TinyORM relationships, one-to-many relationships are defined by defining a hasMany<Related> method on your TinyORM model:

#ifndef POST_H
#define POST_H

#include <orm/tiny/basemodel.hpp>

#include "models/comment.hpp"

using Orm::Tiny::BaseModel;
using Orm::Tiny::Relations::Relation;

class Post final : public BaseModel<Post, Comment>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get the comments for the blog post. */
    std::unique_ptr<Relation<Post, Comment>>
    comments()
    {
        return hasMany<Comment>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "comments")
            relationVisited<Comment>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"comments", &Post::comments},
    };
};

#endif // POST_H

Remember, TinyORM will automatically determine the proper foreign key column for the Comment model. By convention, TinyORM will take the "snake case" name of the parent model and suffix it with _id. So, in this example, TinyORM will assume the foreign key column on the Comment model is post_id.

Once the relationship method has been defined, we can access the QVector<Related *> of related comments by BaseModel's getRelationValue<Related, Container = QVector> method:

#include "models/post.hpp"

auto comments = Post::find(1)->getRelationValue<Comment>("comments");

for (auto *comment : comments) {
    //
}

Since all relationships also serve as query builders, you may add further constraints to the relationship query by calling the comments method and continuing to chain conditions onto the query, only simple where methods are proxied now:

auto comment = Post::find(1)->comments()
                    ->whereEq("title", "foo")
                    .first();

Like the hasOne method, you may also override the foreign and local keys by passing additional arguments to the hasMany method:

return hasMany<Comment>("foreign_key");

return hasMany<Comment>("foreign_key", "local_key");

One To Many (Inverse) / Belongs To

Now that we can access all of a post's comments, let's define a relationship to allow a comment to access its parent post. To define the inverse of a hasMany relationship, define a relationship method on the child model which calls the belongsTo method:

#ifndef COMMENT_H
#define COMMENT_H

#include <orm/tiny/basemodel.hpp>

#include "models/post.hpp"

using Orm::Tiny::BaseModel;
using Orm::Tiny::Relations::Relation;

class Comment final : public BaseModel<Comment, Post>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get the post that owns the comment. */
    std::unique_ptr<Relation<Comment, Post>>
    post()
    {
        return belongsTo<Post>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "post")
            relationVisited<Post>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"post", &Comment::post},
    };
};

#endif // COMMENT_H

Once the relationship has been defined, we can retrieve a comment's parent post by BaseModel's getRelationValue<Related, Tag> method:

#include "models/comment.hpp"

auto comment = Comment::find(1);

return comment->getRelationValue<Post, Orm::One>("post")->getAttribute("title").toString();

In the example above, TinyORM will attempt to find a Post model that has an id which matches the post_id column on the Comment model.

TinyORM determines the foreign key name by examining the type-name of the Related template parameter and suffixing the type-name with a _ followed by the name of the parent model's primary key column. So, in this case, TinyORM assumes that the Post model's foreign key on the comments table is post_id.

However, if the foreign key for your relationship does not follow these conventions, you may pass a custom foreign key name as the first argument to the belongsTo method:

using Orm::Tiny::Relations::Relation;

/*! Get the post that owns the comment. */
std::unique_ptr<Relation<Comment, Post>>
post()
{
    return belongsTo<Post>("foreign_key");
}

If your parent model does not use id as its primary key, or you wish to find the associated model using a different column, you may pass a second argument to the belongsTo method specifying your parent table's custom key:

using Orm::Tiny::Relations::Relation;

/*! Get the post that owns the comment. */
std::unique_ptr<Relation<Comment, Post>>
post()
{
    return belongsTo<Post>("foreign_key", "owner_key");
}

The third belongsTo parameter is the relation name, if you pass it, the foreign key name will be determined from it. By convention, TinyORM will "snake case" this relation name and suffix it with a _ followed by the name of the parent model's primary key column, the __func__ predefined identifier is ideal for this:

/*! Get the post that owns the comment. */
std::unique_ptr<Relation<Comment, Post>>
somePost()
{
    return belongsTo<Post>({}, {}, __func__); // the foreign key will be some_post_id
}

Many To Many Relationships

It will be implemented soon.

Querying Relations

Since all TinyORM relationships are defined via methods, you may call those methods to obtain an instance of the relationship without actually executing a query to load the related models. In addition, all types of TinyORM relationships also serve as query builders, allowing you to continue to chain constraints onto the relationship query before finally executing the SQL query against your database.

For example, imagine a blog application in which a User model has many associated Post models:

#include "models/post.hpp"

using Orm::Tiny::BaseModel;
using Orm::Tiny::Relations::Relation;

class User final : public BaseModel<User, Post>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get all of the posts for the user. */
    std::unique_ptr<Relation<User, Post>>
    posts()
    {
        return hasMany<Post>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "posts")
            relationVisited<Post>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"posts", &User::posts},
    };
};

You may query the posts relationship and add additional constraints to the relationship like so:

#include "models/user.hpp"

auto user = User::find(1);

user->posts()->whereEq("active", 1).get();

You are able to use any of the Laravel query builder's methods on the relationship, so be sure to explore the query builder documentation to learn about all of the methods that are available to you.

So far, not all query builder methods are proxied on the relation base class, only basic where methods are proxied.

Chaining orWhere Clauses After Relationships

As demonstrated in the example above, you are free to add additional constraints to relationships when querying them. However, be careful when chaining orWhere clauses onto a relationship, as the orWhere clauses will be logically grouped at the same level as the relationship constraint:

user->posts()
    ->whereEq("active", 1)
    .orWhere("votes", ">=", 100)
    .get();

The example above will generate the following SQL. As you can see, the or clause instructs the query to return any user with greater than 100 votes. The query is no longer constrained to a specific user:

select *
from posts
where user_id = ? and active = 1 or votes >= 100

In most situations, you should use logical groups to group the conditional checks between parentheses:

user->posts()
    ->where([](auto &query) {
        return query.whereEq("active", 1)
                    .orWhere("votes", ">=", 100);
    })
    .get();

The example above will produce the following SQL. Note that the logical grouping has properly grouped the constraints and the query remains constrained to a specific user:

select *
from posts
where user_id = ? and (active = 1 or votes >= 100)

Relationship Methods

If you do not need to add additional constraints to an TinyORM relationship query, you may access the relationship directly. For example, continuing to use our User and Post example models, we may access all of a user's posts like so:

#include "models/user.hpp"

auto user = User::find(1);

for (auto *post : user->getRelationValue<Post>("posts")) {
    //
}

The getRelationValue<Related> method performs "lazy loading", meaning they will only load their relationship data when you actually access them. Because of this, developers often use eager loading to pre-load relationships they know will be accessed after loading the model. Eager loading provides a significant reduction in SQL queries that must be executed to load a model's relations.

To access eager loaded relationship use BaseModel's getRelation<Related> method:

auto user = User::find(1);

for (auto *post : user->getRelation<Post>("posts")) {
    //
}

As described above TinyORM offers two methods to access relationships; getRelation and getRelationValue.

The getRelation method is for "eager loaded" relations, when the relationship is not loaded, it throws the exception RelationNotLoadedError. The getRelationValue is for "lazy loading", when the relationship is not loaded, it will load it.

Both methods have two overloads, the getRelation<Related, Container = QVector> overload is for obtaining many type relationships:

auto posts = User::find(1)->getRelation<Post>("posts");

The getRelation<Related, Tag> overload is for obtaining "one" type relationships:

auto user = Post::find(1)->getRelation<User, Orm::One>("user");

The same is true for the getRelationValue method.

Eager Loading

When accessing TinyORM relationships by BaseModel's getRelationValue method, the related models are "lazy loaded". This means the relationship data is not actually loaded until you first access them. However, TinyORM can "eager load" relationships at the time you query the parent model. Eager loading alleviates the "N + 1" query problem. To illustrate the N + 1 query problem, consider a Book model that "belongs to" to an Author model:

using Orm::Tiny::BaseModel;
using Orm::Tiny::Relations::Relation;

class Book final : public BaseModel<Book, Author>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get the author that wrote the book. */
    std::unique_ptr<Relation<Book, Author>>
    author()
    {
        return belongsTo<Author>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "author")
            relationVisited<Author>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"author", &Book::author},
    };
};

Now, let's retrieve all books and their authors:

#include <QDebug>

#include "models/book.hpp"

auto books = Book::all();

for (auto &book : books)
    qDebug() << book.getRelationValue<Author, Orm::One>("author")
                    ->getAttribute("name").toString();

This loop will execute one query to retrieve all of the books within the database table, then another query for each book in order to retrieve the book's author. So, if we have 25 books, the code above would run 26 queries: one for the original book, and 25 additional queries to retrieve the author of each book.

Thankfully, we can use eager loading to reduce this operation to just two queries. When building a query, you may specify which relationships should be eager loaded using the with method:

auto books = Book::with("author")->get();

for (auto &book : books)
    qDebug() << book.getRelation<Author, Orm::One>("author")
                    ->getAttribute("name").toString();

For this operation, only two queries will be executed - one query to retrieve all of the books and one query to retrieve all of the authors for all of the books:

select * from books

select * from authors where id in (1, 2, 3, 4, 5, ...)

Eager Loading Multiple Relationships

Sometimes you may need to eager load several different relationships. To do so, just pass a QVector<Orm::WithItem> of relationships to the with method:

auto books = Book::with({{"author"}, {"publisher"}})->get();

Nested Eager Loading

To eager a relationship's relationships, you may use "dot" syntax. For example, let's eager load all of the book's authors and all of the author's personal contacts:

auto books = Book::with("author.contacts")->get();

Eager Loading By Default

Sometimes you might want to always load some relationships when retrieving a model. To accomplish this, you may define a u_with data member on the model:

using Orm::Tiny::BaseModel;
using Orm::Tiny::Relations::Relation;
using Orm::WithItem;

class Book final : public BaseModel<Book, Author>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get the author that wrote the book. */
    std::unique_ptr<Relation<Book, Author>>
    author()
    {
        return belongsTo<Author>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "author")
            relationVisited<Author>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"author", &Book::author},
    };

    /*! The relationships that should always be loaded. */
    QVector<WithItem> u_with {
        {"author"},
    };
};

If you would like to remove an item from the u_with data member for a single query, you may use the without method:

auto books = Book::without("author")->get();

Lazy Eager Loading

Sometimes you may need to eager load a relationship after the parent model has already been retrieved. For example, this may be useful if you need to dynamically decide whether to load related models:

#include "models/book.hpp"

auto book = Book::find(1);

if (someCondition)
    book->load("author");

You may load more relationships at once, to do so, just pass a QVector<Orm::WithItem> of relationships to the load method:

Book::find(1)->load({{"author"}, {"publisher"}});

{note} So far, this only works on models, not on containers returned from BaseModel's get or all methods.

Inserting & Updating Related Models

Recursively Saving Models & Relationships

If you would like to save your model and all of its associated relationships, you may use the push method. In this example, the Post model will be saved as well as its comments and the comment's authors:

auto post = Post::find(1);

post->getRelationValue<Comment>("comments").at(0)->setAttribute("message", "Message");

post->getRelationValue<Comment>("comments").first()
    ->getRelationValue<User, Orm::One>("author")->setAttribute("name", "Author Name");

post->push();

Touching Parent Timestamps

When a model defines a belongsTo relationship to another model, such as a Comment which belongs to a Post, it is sometimes helpful to update the parent's timestamp when the child model is updated.

For example, when a Comment model is updated, you may want to automatically "touch" the updated_at timestamp of the owning Post so that it is set to the current date and time. To accomplish this, you may add a u_touches data member to your child model containing the names of the relationships that should have their updated_at timestamps updated when the child model is updated:

class Comment final : public BaseModel<Comment, Post>
{
public:
    friend class BaseModel;

    using BaseModel::BaseModel;

    /*! Get the post that owns the comment. */
    std::unique_ptr<Relation<Comment, Post>>
    post()
    {
        return belongsTo<Post>();
    }

private:
    /*! The visitor to obtain a type for Related template parameter. */
    void relationVisitor(const QString &relation)
    {
        if (relation == "post")
            relationVisited<Post>();
    }

    /*! Map of relation names to methods. */
    QHash<QString, std::any> u_relations {
        {"post", &Comment::post},
    };

    /*! All of the relationships to be touched. */
    QStringList u_touches {"post"};
};

{note} Parent model timestamps will only be updated if the child model is updated using TinyORM's save, push, or remove method.