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I'm a Software Engineer and a teacher.
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One to many: For example a blog post and comments.
Every blog post can have many comments but each comment only belongs to one blog post.
The way this is implemented is by giving every comment a blog_post_id so you can query the database for every comment with a blog_post_id that equal's the blog post's id.
Many to many: For example tags on a blog post.
Every blog post can have many tags (this one, for example, has two; #orm and #explainlikeimfive), and every tag can be attached to many blog posts (searching #explainlikeimfive will bring up many posts).
This cannot be implemented by adding a blog_post_id attribute to tags, because every tag needs to be able to have many blog_post_ids, instead, it's implemented using a join table.
A join table is a table that has two columns, blog_post_id and tag_id. Every time you want to add a tag to a blog post you add an entry to the join table with the id of the blog post and the id of the tag. Getting a list of all the tags that were applied to a given article is done by querying the join table for all of the tag_ids where the blog_post_id matches your blog post and vice versa for getting a list of all of the articles with a given tag.
This is a very nice answer! I'd just add that in the 'Object' part of ORM, the usage is usually pretty simple. For example, something like blog_post.tags will get the tags associated with a blog post and tag.blog_posts will get the blog posts associated with a given tag.
Behind the scenes the ORM will use its mapping configuration to retrieve these associations from a relational database. That's where a join table is probably going to be used to represent a many-to-many relationship between two tables, e.g. there would be something like an X_BLOG_POST_TAG table that connects the BLOG_POST and TAG tables. However, in your domain code, you usually won't need to be aware of this, which of course is the whole point behind object-relational mapping.
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One to many: For example a blog post and comments.
Every blog post can have many comments but each comment only belongs to one blog post.
The way this is implemented is by giving every comment a
blog_post_idso you can query the database for every comment with ablog_post_idthat equal's the blog post'sid.Many to many: For example tags on a blog post.
Every blog post can have many tags (this one, for example, has two; #orm and #explainlikeimfive), and every tag can be attached to many blog posts (searching #explainlikeimfive will bring up many posts).
This cannot be implemented by adding a
blog_post_idattribute to tags, because every tag needs to be able to have manyblog_post_ids, instead, it's implemented using a join table.A join table is a table that has two columns,
blog_post_idandtag_id. Every time you want to add a tag to a blog post you add an entry to the join table with the id of the blog post and the id of the tag. Getting a list of all the tags that were applied to a given article is done by querying the join table for all of thetag_ids where theblog_post_idmatches your blog post and vice versa for getting a list of all of the articles with a given tag.This is a very nice answer! I'd just add that in the 'Object' part of ORM, the usage is usually pretty simple. For example, something like
blog_post.tagswill get the tags associated with a blog post andtag.blog_postswill get the blog posts associated with a given tag.Behind the scenes the ORM will use its mapping configuration to retrieve these associations from a relational database. That's where a join table is probably going to be used to represent a many-to-many relationship between two tables, e.g. there would be something like an
X_BLOG_POST_TAGtable that connects theBLOG_POSTandTAGtables. However, in your domain code, you usually won't need to be aware of this, which of course is the whole point behind object-relational mapping.