M2M without on_conflict

[Forceres]

I think that on_conflict part in add_m2m would be very useful for avoiding duplicates in m2m, why it isn't supported yet?

[sinisaos]

@Forceres There was a PR for it, but it never took off. Maybe @dantownsend has a better way to solve that problem. As you can see in that PR, it's just a few lines of code (with combination of unique constraints in joining_table) that would be useful to prevent duplicate entries in M2M.

[Forceres]

@Forceres There was a PR for it, but it never took off. Maybe @dantownsend has a better way to solve that problem. As you can see in that PR, it's just a few lines of code (with combination of unique constraints in joining_table) that would be useful to prevent duplicate entries in M2M.

A lot of vital PRs were not merged. What a pity!