Rails creating a M:M relationship - ruby-on-rails

Question

I have a user and team models joined by a membership model.
One user can have many teams and each team can have many users etc.
class User < ApplicationRecord
has_many :memberships
has_many :teams, through: :memberships
End
class Team < ApplicationRecord
has_many :memberships
has_many :users, through: :memberships
End
class Membership < ApplicationRecord
belongs_to :user
belongs_to :team
end
I am looking for a way to simply create a memberships record however am missing something:
2.4.0 :026 > t = Team.last
Team Load (1.2ms) SELECT "teams".* FROM "teams" ORDER BY "teams"."id" DESC LIMIT $1 [["LIMIT", 1]]
=> #<Team id: 42129, name: "Reds", description: "A good team", created_at: "2017-05-18 05:05:09", updated_at: "2017-05-18 05:05:09">
2.4.0 :027 > User.first.memberships
User Load (1.7ms) SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1 [["LIMIT", 1]]
Membership Load (1.1ms) SELECT "memberships".* FROM "memberships" WHERE "memberships"."user_id" = $1 LIMIT $2 [["user_id", 1], ["LIMIT", 11]]
=> #<ActiveRecord::Associations::CollectionProxy []>
2.4.0 :028 > User.first.memberships << t
User Load (0.8ms) SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1 [["LIMIT", 1]]
(0.4ms) BEGIN
(0.4ms) ROLLBACK
ActiveRecord::AssociationTypeMismatch: Membership(#23507760) expected, got #<Team id: 42129, name: "Reds", description: "A good team", created_at: "2017-05-18 05:05:09", updated_at: "2017-05-18 05:05:09"> which is an instance of Team(#33684520)
from (irb):28
2.4.0 :029 >
What am I missing here?
Update:
When I create a new memberships record and manually add the fk values I get this error when I try to save:
2.4.0 :037 > m
=> #<Membership id: nil, user_id: 1, team_id: 22641, created_at: nil, updated_at: nil>
2.4.0 :038 > m.save
(0.5ms) BEGIN
(0.4ms) ROLLBACK
NoMethodError: undefined method `class_name' for nil:NilClass
Did you mean? class_eval
from (irb):38
Update2
2.4.0 :022 > user = User.first
User Load (2.7ms) SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1 [["LIMIT", 1]]
=> #<User id: 1, created_at: "2017-05-15 08:17:01", updated_at: "2017-05-19 02:54:30">
2.4.0 :023 > team = Team.first
Team Load (2.3ms) SELECT "teams".* FROM "teams" ORDER BY "teams"."id" ASC LIMIT $1 [["LIMIT", 1]]
=> #<Team id: 22641, name: "Reds", description: "This is a good team", created_at: "2017-05-18 01:41:00", updated_at: "2017-05-18 05:05:09">
2.4.0 :027 > Membership.delete_all
SQL (1.9ms) DELETE FROM "memberships"
=> 1
2.4.0 :029 > m=Membership.new(user: user, team: team)
=> #<Membership id: nil, user_id: 1, team_id: 22641, created_at: nil, updated_at: nil>
2.4.0 :031 > m.save
(1.0ms) BEGIN
SQL (2.3ms) INSERT INTO "memberships" ("user_id", "team_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id" [["user_id", 1], ["team_id", 22641], ["created_at", "2017-05-22 03:37:22.803718"], ["updated_at", "2017-05-22 03:37:22.803718"]]
(2.1ms) COMMIT
=> true
2.4.0 :032 > Membership.delete_all
SQL (1.9ms) DELETE FROM "memberships"
=> 1
2.4.0 :033 > m=Membership.new
=> #<Membership id: nil, user_id: nil, team_id: nil, created_at: nil, updated_at: nil>
2.4.0 :034 > m.user_id=user.id
=> 1
2.4.0 :035 > m.team_id=team.id
=> 22641
2.4.0 :036 > m
=> #<Membership id: nil, user_id: 1, team_id: 22641, created_at: nil, updated_at: nil>
2.4.0 :037 > m.save
(0.5ms) BEGIN
User Load (0.9ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2 [["id", 1], ["LIMIT", 1]]
(0.8ms) ROLLBACK
NoMethodError: undefined method `class_name' for nil:NilClass
Did you mean? class_eval
from (irb):37

Answer 1

The first error message you're getting (before the Update) is because ActiveRecord is expecting an instance of Membership, and it's getting a Team. User.first.teams << t should work for that example.
As far as the error you shared in the Update, I didn't encounter that when I tried it, and it seemed to work fine with the relationships you gave.
2.2.3 :009 > user = User.first
User Load (0.2ms) SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1
=> #<User id: 1>
2.2.3 :010 > team = Team.first
Team Load (0.2ms) SELECT "teams".* FROM "teams" ORDER BY "teams"."id" ASC LIMIT 1
=> #<Team id: 1>
2.2.3 :011 > membership = Membership.new(user: user, team: team)
=> #<Membership id: nil, user_id: 1, team_id: 1>
2.2.3 :012 > membership.save
(0.1ms) begin transaction
SQL (1.2ms) INSERT INTO "memberships" ("user_id", "team_id") VALUES (?, ?) [["user_id", 1], ["team_id", 1]]
(2.3ms) commit transaction
=> true
One thing to double check would be in how you're constructing the Membership record, and that there are no pluralization errors in the models. Passing in the actual User and Team instances to Membership.new or Membership.create should work.