how to have relations many to many in redis

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With Redis, relationships are typically represented by sets. A set can be used
to represent a one-way relationship, so you need one set per object to
represent a many-to-many relationship.

It is pretty useless to try to compare a relational database model to Redis
data structures. With Redis, everything is stored in a denormalized way. Example:

# Here are my categories
> hset category:1 name cinema  ... more fields ...
> hset category:2 name music   ... more fields ...
> hset category:3 name sports  ... more fields ...
> hset category:4 name nature  ... more fields ...

# Here are my users
> hset user:1 name Jack   ... more fields ...
> hset user:2 name John   ... more fields ...
> hset user:3 name Julia  ... more fields ...

# Let's establish the many-to-many relationship
# Jack likes cinema and sports
# John likes music and nature
# Julia likes cinema, music and nature

# For each category, we keep a set of reference on the users
> sadd category:1:users 1 3
> sadd category:2:users 2 3
> sadd category:3:users 1
> sadd category:4:users 2 3

# For each user, we keep a set of reference on the categories
> sadd user:1:categories 1 3
> sadd user:2:categories 2 4
> sadd user:3:categories 1 2 4

Once we have this data structure, it is easy to query it using the set algebra:

# Categories of Julia
> smembers user:3:categories
1) "1"
2) "2"
3) "4"

# Users interested by music
> smembers category:2:users
1) "2"
2) "3"

# Users interested by both music and cinema
> sinter category:1:users category:2:users
1) "3"

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