Junction table in PostgreSQL

Question

I have two tables

recipe table
-----------------------
|recipe_id   |name  |
-----------------------
|1           |Pasta |
|2           |Pizza |
|3           |Fish  |


ingredient table
-------------------------------
|ingredient_id   |name        |
-------------------------------
|1               |Spaghetti   |
|2               |Salmon      |
|3               |Tomato sauce|

I created a junction table called "recipes_ingredients"

-------------------------------
|id   |fk_recipe|fk_ingredient|
-------------------------------
|1    |1        |1           |
|2    |2        |3           |
|3    |3        |2           |

My first question is: is it correct how I created the "recipes_ingredients" table? Do I need the "id" or I can just have the two foreign keys "fk_recipe" and "fk_ingredient"?

And the second one is if it's possible to autopopulate the junction table when I insert a record in the other two tables. Or do I have to add manually every single association in the junction table?

Answer 1

My first question is: is it correct how I created the "recipes_ingredients" table? Do I need the "id" or I can just have the two foreign keys "fk_recipe" and "fk_ingredient"?

Your junction table looks great! Your ID in recipes_ingredients will be auto numbered. I'd rename fk_recipe to recipe_id and fk_ingredient to ingredient_id , but it's totally fine to keep it the way you have it too. It's preference based.

And the second one is if it's possible to autopopulate the junction table when I insert a record in the other two tables. Or do I have to add manually every single association in the junction table?

Typically, you would manually enter ingredients that you don't already have in ingredients table, enter a new recipe name, and finally add entries in junction table.

However, if you have rules that say: All recipes containing the word pizza will have ingredient 1, 2 and 3, then you can create a stored procedure with logic to add information in the junctions table based on your rules. You can't cover all your use cases using a stored procedure, so you would still have to go to the junction table and add some entries manually.

Typically, web developers create a web page that allow such interaction of creating a recipe using a web page and then allowing selections of (or drag drop of) ingredients. In the background, the web page updates the junction table.

Answer 2

It would be better if the junction table had no id column. The primary key should be on (fk_recipe, fk_ingredient) – this should be a unique and not NULL anyway.

You'll have to add elements to the junction table explicitly - just because you add a new recipe, it is not yet clear which ingredients are required. The junction table should only have an entry if a certain ingredient is needed for a certain recipe.

The junction table can and should include additional data, like the amount of the ingredient that is required.