this post was submitted on 16 Jun 2024
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understanding many2one, one2many and many2many relationshit (lemmy.ca)
submitted 5 months ago by [email protected] to c/[email protected]
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Hii,

I am new to database thing so I am trying to wrap my head around it.

  1. many2one: so in this relationship you will have more than one record in one table which matches to only one record in another table. something like A <-- B. where (<--) is foreign key relationship. so B will have a column which will be mapped to more than one record of A.

  2. one2many: same as many2one but instead now the foreign key constrain will look something like A --> B.

  3. many2many: this one is interesting because this relationship doesn't make use of foreign key directly. to have this relationship between A and B you have to make a third database something like AB_rel. AB_rel will hold values of primary key of A and also primary key of B. so that way we can map those two using AB_rel table.

tell me if I got something wrong :) give me some suggestion <3

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[–] p_consti 26 points 5 months ago

Usually we don't distinguish between many2one and one2many, since it's the same just viewed from the other entity.

There is one more class though, which is one2one. That is, the entities have a direct relationship. Sometimes this also includes the case where you have zero or one, i.e. the relation is optional on one side. This can be accomplished with an FK plus unique constraint or by merging the tables.