ruby on rails - The has_many :through Association - Modification? -

i have read through basics of association app fits `has_many :through associations properly...but there 1 issue other question lacks.

i'd rather ask question keep question clear , clean.

in understanding of how has_many :through associations work whenever doctor has new patient, new appointment created or has created patient. while makes sense, my app dont 1 bit.

in case, whenever user 'likes' someone's posts, creates new post. dont want 1 bit. i'm trying understanding of rails association answer other question pertaining one. how prevent 'appointment' being automatically created? or make perfect sense, when new instance of patient created , merged doctor.patients, creates new appointment.

d = doctor.new p = patient.new = appointment.new # created or not  foo = doctor.find(1) foo.patients << p # creates new appointment not want. 

whenever there many-to-many relationship, instance:

• any single doctor can have multiple patients
• any single patient can have multiple doctors

the database needs third table. because can't put multiple values patient_id column of doctor table, , can't add on columns (how know how many add, , searching nightmare!).

the solution create "join" table, table 2 columns—in case doctor_id , patient_id. if doctor "x" has patients "a", "b", , "c", can add 3 records join table. if patient "a" has 2 doctors, "x" , "y," can represent well:

example join table "doctors_patients"

doctor_id | patient_id     x               x           b     x           c     y           

see how gets around problem? in example cited, appointment table, although it's model, acts join table doctors , patients. there's no reason can't add additional columns such date, time, room number, etc. on join table.

example join table "appointments"

doctor_id | patient_id |  date     x                  1/1/2016     x           b        1/2/2016     x           c        1/3/2016     y                  1/4/2016 

but if there's no need refer model representing join table, can use has_and_belongs_to_many association (see rails guide). still need make join table in migration regardless, again, need in order represent many-to-many association, there's no reason have give name or represent model.

models:

class user < activerecord::base   has_and_belongs_to_many :posts end  class post < activerecord::base   has_and_belongs_to_many :users end 

migration:

class createusersandposts < activerecord::migration   def change     create_table :users |t|       t.string :name       t.timestamps null: false     end      create_table :posts |t|       t.text :content       t.timestamps null: false     end      # join table     create_table :posts_users, id: false |t| # posts comes before users alphabetically       t.belongs_to :posts, index: true       t.belongs_to :users, index: true     end   end end