Demo of two ways to pass locals to an ERB template.

erb_rendering.rb

require 'erb'
require 'ostruct'

class TemplateContext
  def initialize(locals_hash)
    locals_hash.each do |key, value|
      singleton_class.send(:define_method, key) { value }
    end
  end

  def bound_locals
    binding
  end
end

class ExampleContext
  def template
    'foo=<%= foo %> bar=<%= bar %> baz=<%= baz %>'
  end

  def good
    good = TemplateContext.new(foo: 'foo', bar: 'bar', baz: 'baz')
    ERB.new(template).result(good.bound_locals)
  end

  def missing
    missing  = TemplateContext.new(foo: 'foo', bar: 'bar')
    ERB.new(template).result(missing.bound_locals)
  end

  def shadowed
    baz = 'shadowed'
    good = TemplateContext.new(foo: 'foo', bar: 'bar', baz: 'baz')
    ERB.new(template).result(good.bound_locals)
  end
end

class ExampleOStruct
  def template
    'foo=<%= foo %> bar=<%= bar %> baz=<%= baz %>'
  end

  def good
    good = OpenStruct.new(foo: 'foo', bar: 'bar', baz: 'baz').instance_eval{binding}
    ERB.new(template).result(good)
  end

  def missing
    missing  = OpenStruct.new(foo: 'foo', bar: 'bar').instance_eval{binding}
    ERB.new(template).result(missing)
  end

  def shadowed
    baz = 'shadowed'
    shadowed = OpenStruct.new(foo: 'foo', bar: 'bar', baz: 'baz').instance_eval{binding}
    ERB.new(template).result(shadowed)
  end
end

# Using OStruct can have some bugs...
ExampleOStruct.new.good # => 'foo=foo bar=bar baz=baz'

ExampleOStruct.new.missing # => 'foo=foo bar=bar baz='

ExampleOStruct.new.shadowed # => 'foo=foo bar=bar baz=shadowed'

# That the explicit context doesn't
ExampleContext.new.good # => 'foo=foo bar=bar baz=baz'

ExampleContext.new.missing # => raises error

ExampleContext.new.shadowed # => 'foo=foo bar=bar baz=baz'