has_many question

rubyonrails-talk
1

Hi all,

I need some guidance please.

I have several tables related by "has_many" and I would like to find the "distant children".

Tables: Subjects Books Chapters Sections Subsections Minisections

They are all related heirarchichally, so book for example is related to subjects and chapters the following way. All are related the same way. Book belongs_to :subject has_many :chapters

So if I have the subject id, how can I find all the Minisections belonging to that subject?

Thanks, Dave

2

Dave Castellano wrote in post #1126985:

Hi all,

I need some guidance please.

I have several tables related by "has_many" and I would like to find the "distant children".

Tables: Subjects Books Chapters Sections Subsections Minisections

They are all related heirarchichally, so book for example is related to subjects and chapters the following way. All are related the same way. Book belongs_to :subject has_many :chapters

So if I have the subject id, how can I find all the Minisections belonging to that subject?

Database normalization is a good thing... until it isn't. I fear you've run into one of those situations where de-normalization might be very useful for the sake of efficiency.

Instead of doing a fully hierarchical structure you could flatten parts of it. For example your Minisection could have id, subject_id, book_id, chapter_id, section_id, subsection_id and then:

Minisection   belongs_to :subject   belongs_to :book   belongs_to :chapter   belongs_to :section   belongs_to :subsection

Now you can find any list mini-section without using any joins in any way you see fit.

minisections = Minisection.where({ subject: my_subject, book: my_book, chapter: my_chapter, section: my_section, subsection: my_subsection }

3

Thanks Robert,

Unfortunately, a minisection can belong to many books, chapters, ect... so needs a join table and can't be flattened.

Dave

4

Please quote the previous message so that the thread can be followed. This is a mailing list not a forum, though you may be accessing it via a forum like interface

Unfortunately, a minisection can belong to many books, chapters, ect... so needs a join table and can't be flattened.

That does not make sense, in your original post you implied minisection belongs_to subsection, and so on up through the heirarchy. Therefore any particular minisection is associated with only one subsection, section, chapter and so on.

Colin

5

Posted by Colin Law (Guest) on 2013-11-12 14:56 Please quote the previous message so that the thread can be followed. This is a mailing list not a forum, though you may be accessing it via a forum like interface

Thanks. Did not know this. I see it in the form of a forum.

Unfortunately, a minisection can belong to many books, chapters, ect...

so needs a join table and can't be flattened.

Posted by Colin Law (Guest) on 2013-11-12 14:56 That does not make sense, in your original post you implied minisection belongs_to subsection, and so on up through the heirarchy. Therefore any particular minisection is associated with only one subsection, section, chapter and so on.

So, original question... Having several tables related by "has_many" and would like to find the "distant children". eg: Tables: Subjects Books Chapters Sections Subsections Minisections

They are all related heirarchichally, so book for example is related to subjects and chapters the following way. All are related the same way. Book belongs_to :subject has_many :chapters

So if I have the subject id, how can I find all the Minisections belonging to that subject without adding all related table id's in each table?

6

The below example allows you to jump 2 levels below 'Subject'. Probably you can try to nest these (I cannot test this right now):

class Subject   has_many :books   has_many :chapters, through: :books end

now subject.chapters should work. Try going further down the hierarchy

Simeon

7

Simeon Manolov wrote in post #1127415:

The below example allows you to jump 2 levels below 'Subject'. Probably you can try to nest these (I cannot test this right now):

class Subject   has_many :books   has_many :chapters, through: :books end

now subject.chapters should work. Try going further down the hierarchy

Simeon

Thank you. I'll give it a try.

Dave