Rails 4 has_many through: nested model id field is not inserted - ruby-on-rails-4

Question

I have visit and condition_value models, associated with has_many though by condition_visit.
Also, there are two other models: condition_type and condition_name. So basically there are a few conditions, each has a type and several values. When visit is created, the user chooses several values for every condition. There is also a details field for every condition.
I omitted the parts of code that seemed irrelevant.
class ConditionValue < ActiveRecord::Base
belongs_to :condition_name
has_many :visits
has_many :condition_visits, through: :visits
end
class ConditionVisit < ActiveRecord::Base
belongs_to :visit
belongs_to :condition_value
end
class Visit < ActiveRecord::Base
has_many :condition_values, through: :condition_visits
has_many :condition_visits, :dependent => :destroy
accepts_nested_attributes_for :condition_visits, :allow_destroy => true
end
controller:
class VisitsController < ApplicationController
...
def new
#visit = Visit.new
#visit.condition_visits.build
end
...
private
# Use callbacks to share common setup or constraints between actions.
def set_visit
#visit = Visit.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def visit_params
params.require(:visit).permit( ... , condition_visits_attributes:
[:details, condition_value_id:[]])
end
end
view:
<div class="field">
<% ConditionType.all.each do |type| %>
<% ConditionName.all.each do |v| %>
<% if ConditionName.find_by(:name => v.name, :condition_type_id => type.id) %>
<%= f.fields_for :condition_visits do |s| %>
<%= v.name %> <%= s.collection_select(:condition_value_id, ConditionName.find_by(:name => v.name, :condition_type_id => type.id).condition_values, :id, :name, {:include_hidden => false}, :multiple => true) %>
Details: <%= s.text_field :details %>
<% end %>
<% end %>
<br>
<% end %>
<hr>
<% end %>
The records are inserted into the DB, but only the details and visit_id fields, condition_value_id is NULL.
Log:
Parameters: {"utf8"=>"✓", "authenticity_token"=>"eo9jxQqPk0UlPxAomDy4Njvf2+evVE/yySiaDFZMQM4Db5IvmTQ5fDK8wtxd5bTDMNfzfDBxXO+c+RdAXMjiMQ==",
"visit"=>{"condition_visits_attributes"=>{
"0"=>{"condition_value_id"=>["2", "3"], "details"=>"1"},
"1"=>{"condition_value_id"=>["13"], "details"=>"2"},
"2"=>{"details"=>""}, "3"=>{"details"=>""},
"4"=>{"details"=>""}, "5"=>{"details"=>""}, "6"=>{"details"=>""},
"7"=>{"details"=>""}, "8"=>{"details"=>""}, "9"=>{"details"=>""},
"10"=>{"details"=>""}, "11"=>{"details"=>""}, "12"=>{"details"=>""},
"13"=>{"details"=>""}, "14"=>{"details"=>""}, "15"=>{"details"=>""},
"16"=>{"details"=>""}, "17"=>{"details"=>""}, "18"=>{"details"=>""},
"19"=>{"details"=>""}, "20"=>{"details"=>""}, "21"=>{"details"=>""},
"22"=>{"details"=>""}, "23"=>{"details"=>""}, "24"=>{"details"=>""},
"25"=>{"details"=>""}, "26"=>{"details"=>""}, "27"=>{"details"=>""},
"28"=>{"details"=>""}, "29"=>{"details"=>""}}, ...,
"commit"=>"Create Visit", "doctor_id"=>"1", "patient_id"=>"14"}
....
SQL (0.2ms) INSERT INTO "condition_visits" ("details", "visit_id",
"created_at", "updated_at") VALUES (?, ?, ?, ?) [["details", "1"],
["visit_id", 236], ["created_at", "2016-03-22 13:20:46.254311"],
["updated_at", "2016-03-22 13:20:46.254311"]]
SQL (0.1ms) INSERT INTO "condition_visits" ("details", "visit_id",
"created_at", "updated_at") VALUES (?, ?, ?, ?) [["details", "2"],
["visit_id", 236], ["created_at", "2016-03-22 13:20:46.256021"],
["updated_at", "2016-03-22 13:20:46.256021"]]
SQL (0.1ms) INSERT INTO "condition_visits" ("details", "visit_id",
"created_at", "updated_at") VALUES (?, ?, ?, ?) [["details", ""],
["visit_id", 236], ["created_at", "2016-03-22 13:20:46.257165"],
["updated_at", "2016-03-22 13:20:46.257165"]]
...
How can I fix this?