Custom validation causing other validations to fail - ruby-on-rails

Question

Title pretty much explains the problem.
Here were my validations for my model:
class Lesson < ActiveRecord::Base
belongs_to :school
has_many :users, through: :goals
has_many :goals, dependent: :destroy
validates :school_id, presence: true
validates :date, presence: true, uniqueness: { scope: :school_id }
validates :attendance, numericality: { only_integer: true,
greater_than: 0 },
presence: true, if: :finished?
validates :lesson_plan_week_number, numericality: { only_integer: true,
greater_than: 0 },
presence: true, if: :finished?
end
All of my model tests (aside from the ones dealing with the custom validation) pass with the above code
Then I added validate :motion_presence and the following private method:
def motion_validation
if debate?
errors.add(:motion, "must be present if a debate was held")
else
errors.add(:motion, "cannot be present if a debate was held")
end
end
and now the tests that once passed are failing. What's happening?
Here's the relevant code in the model if I'm organizing it incorrectly
class Lesson < ActiveRecord::Base
belongs_to :school
has_many :users, through: :goals
has_many :goals, dependent: :destroy
validates :school_id, presence: true
validates :date, presence: true, uniqueness: { scope: :school_id }
validates :attendance, numericality: { only_integer: true,
greater_than: 0 },
presence: true, if: :finished?
validates :lesson_plan_week_number, numericality: { only_integer: true,
greater_than: 0 },
presence: true, if: :finished?
validate :motion_presence
private
# Validates the motion depending on debate?
def motion_presence
if debate?
errors.add(:motion, "must be present if a debate was held")
else
errors.add(:motion, "cannot be present if a debate was held")
end
end
end

Answer 1

def motion_presence
if debate?
errors.add(:motion, "must be present if a debate was held")
else
errors.add(:motion, "cannot be present if a debate was held")
end
end
This will always fail because you added to errors in either case.
If you wanted to fail it for multiple conditions, then try doing it in elsif. else means EVERYTHING ELSE and its failing always.
Do it like this:
def motion_presence
if debate?
errors.add(:motion, "must be present if a debate was held")
elsif some_condition
errors.add(:motion, "cannot be present if a debate was held")
end
end

Answer 2

That is because in any case whether debate? returns true or false, it is adding errors to the motion so it will never be successful which results in not saving the lesson object as errors are already added for motion field.
I think you need to add a case where it should just return without adding any errors. Besides just add errors for specific cases