Trouble writing a method - ruby-on-rails

Question

I'm trying to write a definition to use as the options for a select field in a form.
Miniature.rb
has_many :manufacturers
has_many :lines
end
Line.rb
belongs_to :manufacturer
end
When adding a #miniature to a #line I want the drop down to only display the #lines that belong_to a #manufacturer that corresponds with that #miniature.
This is my attempt
def mfr_lines
#mfrs = #miniature.manufacturers.map
#mfr_lines = #mfrs.lines.map
end
The first line seems to work, it combines all the #miniature.manufacturers
What I need to do then is get all the lines for EACH #manufacturer and join them into one array. I can't work out how to do the EACH bit or how to join them. Tried a number of possible solutions to no avail. Any help much appreciated.
It wants to work ultimately like this in the view
<%= f.select :line_id, #mfr_lines, include_blank: true %>

Answer 1

#hoffm's solution is good, but a little improved version can be used:
def mfr_lines
#lines = Line.where(manufacturer_id: #miniature.manufacturers.pluck(:id))
end
And its usage for the select with options_for_select (1):
f.select :line_id, options_for_select(#lines.map{|l| [l.name, l.id] }), include_blank: true
Why is it better than #miniature.manufacturers.map(&:lines).flatten?
manufacturers.map(&:lines) actually call the method .lines (triggers a SQL query) on each manufacturer. If you have 10 of them, it will call 10 times SELECT * FROM lines WHERE lines.manufacturer_id = <id>,
Line.where(manufacturer_id: #miniature.manufacturers.pluck(:id)) is and will always be only 2 combined queries:
# #miniature.manufacturers.pluck(:id) (2)
SELECT id FROM manufacturers WHERE manufacturers.miniature_id = <#miniature.id> ;
# This returns a list of IDs
# Line.where(manufacturer_id: list_of_ids)
SELECT * FROM lines WHERE lines.manufacturer_id IN (<list_of_ids>) ;
In overall, the following Ruby on Rails code:
Line.where(manufacturer_id: #miniature.manufacturers.pluck(:id))
Triggers the following SQL:
SELECT *
FROM lines
WHERE lines.manufacturer_id
IN ( SELECT id
FROM manufacturers
WHERE manufacturers.miniature_id = <#miniature.id> )
References:
(1) options_for_select documentation
(2) .pluck documentation

Answer 2

Calling map without a block returns an enumerator, which it doesn't sound like you were expecting. If I understand what you're after, this is the way to go:
def mfr_lines
#miniature.manufacturers.map(&:lines).flatten
end
This code retrieves the array of all manufacturers associated with the miniature, and then maps each of those to an array of their lines. The result is an array of arrays of lines, which we then flatten to get an array of lines over which you can iterate.