Database Structure involving dynamic fields

Question

Im working on a project. Its mostly for learning purposes, i find actually trying a complicated project is the best way to learn a language after grasping the basics. Database design is not a strong point, i started reading up on it but its early days and im still learning.

Here is my alpha schema, im really at the point where im just trying to jot down everything i can think of and seeing if any issues jump out. http://diagrams.seaquail.net/Diagram.aspx?ID=10094#

Some of my concerns i would like feedback on:

Notice for the core attributes like area for example, lets say for simplicity the areas are kitchen,bedroom,garden,bathroom and living room. For another customer that might be homepage,contact page,about_us,splash screen. It could be 2 areas and it could be 100, there isn't a need to limit it.

I created separate tables for the defaults and each is linked to a bug. Later i came to the problem of custom fields, if someone wants for example to mark which theme the bug applies to we dont have that, there is probably a 100 other things so i wanted to stick to a core set of attributes and the custom fields give people flexibility.

However when i got to the custom fields i knew i had an issue, i cant be creating a table for every custom field so i instead used 2 tables. custom fields and custom_field_values. The idea is every field including defaults would be stored in this table and each would be linked to the values table which would just have something like this

custom_fields table
id            project_id        name
01            1                 area(default)
12            2                 rooms(custom)
13            4                 website(custom)

custom_field_values table
id            area           project_id    sort_number
667           area1          1             1
668           area2          1             2
669           area3          1             3
670           area4          1             4
671           bedroom        2             1
672           bathroom       2             2
673           garden         2             3
674           livingroom     2             4
675           homepage       4             1
676           about_us       4             2
677           contact        4             3
678           splash page    4             4

Does this look like an efficient way to handle dynamic fields like this or is there other alternatives?

The defaults would be hard coded so you can either use them or replace with your own or i could create a another table to allow users to edit the name of the defaults which would be linked to their project. Any feedback is welcome and if there something very obvious with issues in the scheme please feel free to critique.

Answer 1

You have reinvented an old antipattern called Entity-Attribute-Value. The idea of custom fields in a table is really logically incompatible with a relational database. A relation has a fixed number of fields.

But even though it isn't properly relational, we still need to do it sometimes.

There are a few methods to mimic custom fields in SQL, though most of them break rules of normalization. For some examples, see:

• Product table, many kinds of product, each product has many parameters on StackOverflow
• My presentation Extensible Data Modeling with MySQL
• My book SQL Antipatterns: Avoiding the Pitfalls of Database Programming