What other attribute should i add for my Gorm table?

Question

My goal is to simply join three tables (Sorry my knowledge in sql is limited)

I'm building a community-based platform like Patreon where each community can have different plans (Free, Paid)

Example Code:

type User struct {
    Communities []Community `gorm:"many2many:user_communities" json:"communities"`
}

type Community struct {
    UserID `json:"userID"`
    Users []User `gorm:"many2many:user_communities" json:"users"`
    Plans []Plan `json:"plans"`
    // How do i add another attirbute to check for user with different plan?
}

type Plan struct {
    CommunityID uint `json:"communityID"`
    Name string `json:"name"`
    Price string `json:"price"`
}

Based on the model above

1. One user can join many communities
2. One community can have many users
3. One user can own a community (creator)
4. and for each community, one user can join for the different plans (Free, Paid)

I'm more focused on how to solve number 4

So the question here is what should I add for my community table to differentiate between the user who joins as Free plan or Paid plan?

because I can only query users who belong to a specific community not users who belong to that community based on a Free or Paid plan.

Example query

// Find whether that user belongs to a community
if err := database.Mysql.Table("users u").Joins("JOIN user_communities uc ON u.id = uc.user_id").Where("u.id = ? AND uc.community_id = ?", userID, communityID).Select("1").Scan(&found).Error; err != nil {
    c.JSON(http.StatusBadRequest, gin.H{"error": "error happened"})
    return
}

Answer 1

One way could be to make the plan part of the many-to-many table.

I don't know gorm really, but in plain SQL this could look something like this.

schema:

create table appUser (
  id serial not null,
  name text not null,
  primary key (id)
);


create table Community (
  id serial not null,
  owner_id int not null,
  name text not null,
  primary key (id),
  foreign key (owner_id) references appUser(id)
);

create table plan (
  id serial not null,
  community_id int not null,
  name text not null,
  price float not null,
  primary key (id),
  foreign key (community_id) references Community(id)
);


create table UserCommunity (
  user_id int not null,
  community_id int not null,
  plan_id int not null,
  primary key (user_id, community_id),
  foreign key (plan_id) references plan(id)
);

usage:

insert into appUser (name) values ('maria'), ('jose'), ('clara');

insert into Community (name, owner_id) values ('community1', 1);

insert into plan (community_id, name, price) values 
(1, 'free', 0.0), (1, 'paid', 10.0);

insert into UserCommunity (user_id, community_id, plan_id) values
  (2, 1, 1), (3, 1, 2);

select
  appUser.name as "user",
  community.name as "community",
  plan.name as "plan"
from appUser
  join UserCommunity on appUser.id = UserCommunity.user_id
  join Community on Community.id = UserCommunity.community_id
  join plan on plan.id = UserCommunity.plan_id
where plan.name = 'paid';

Fiddle: https://dbfiddle.uk/?rdbms=postgres_13&fiddle=bd37832969396a40944f40ef17f18465