class Traject::MarcExtractor

MarcExtractor is a class for extracting lists of strings from a MARC::Record, according to specifications. See Traject::MarcExtractor::Spec for description of string string arguments used to specify extraction. See initialize for options that can be set controlling extraction.

Examples:

array_of_stuff   = MarcExtractor.new("001:245abc:700a").extract(marc_record)
values           = MarcExtractor.new("245a:245abc").extract_marc(marc_record)
seperated_values = MarcExtractor.new("020a:020z").extract(marc_record)
bytes            = MarcExtractor.new("008[35-37]")

## String extraction specifications

Extraction directions are supplied in strings, usually as the first parameter to MarcExtractor.new or MarcExtractor.cached. These specifications are also the first parameter to the marc_extract macro.

A String specification is a string (or array of strings) which consists of one or more Data and Control Field Specifications seperated by colons.

A Data Field Specification is of the form:

indicator spec must be two chars, but one can be * meaning “don't care”. space to mean 'blank'

"245|01|abc65:345abc:700|*5|:800"

A Control Field Specification is used with tags for control (fixed) fields (ordinarily fields 001-010) and includes a tag and a a byte slice specification.

"008[35-37]:007[5]""
=> bytes 35-37 inclusive of any field 008, and byte 5 of any field 007

## Subfield concatenation

Normally, for a spec including multiple subfield codes, multiple subfields from the same MARC field will be concatenated into one string separated by spaces:

600 a| Chomsky, Noam x| Philosophy.
600 a| Chomsky, Noam x| Political and social views.
MarcExtractor.new("600ax").extract(record)
# results in two values sent to Solr:
"Chomsky, Noam Philosophy."
"Chomsky, Noam Political and social views."

You can turn off this concatenation and leave individual subfields in seperate strings by setting the `separator` option to nil:

MarcExtractor.new("600ax", :separator => nil).extract(record)
# Results in four values being sent to Solr (or 3 if you de-dup):
"Chomksy, Noam"
"Philosophy."
"Chomsky, Noam"
"Political and social views."

However, **the default is different for specifications with only a single subfield**, these are by default kept seperated:

020 a| 285197145X a| 9782851971456
MarcExtractor.new("020a:020z").extract(record)
# two seperate strings sent to Solr:
"285197145X"
"9782851971456"

For single subfield specifications, you force concatenation by repeating the subfield specification:

MarcExtractor.new("020aa:020zz").extract(record)
# would result in a single string sent to solr for
# the single field, by default space-separated:
"285197145X 9782851971456"

## Note on Performance and MarcExtractor creation and reuse

A MarcExtractor is somewhat expensive to create, and has been shown in profiling/ benchmarking to be a bottleneck if you end up creating one for each marc record processed. Instead, a single MarcExtractor should be created, and re-used per MARC record.

If you are creating a traject 'macro' method, here's one way to do that, capturing the MarcExtractor under closure:

def some_macro(spec, other_args, whatever)
  extractor = MarcExtractor.new( spec )
  # ...
  return lambda do |record, accumulator, context|
     #...
     accumulator.concat extractor.extract(record)
     #...
  end
end

In other cases, you may find it convenient to improve performance by using the MarcExtractor#cached method, instead of MarcExtractor#new, to lazily create and then re-use a MarcExtractor object with particular initialization arguments.

Constants

ALTERNATE_SCRIPT_TAG
EMPTY
SPACE
SUBFIELD_6

Attributes

options[RW]
spec_set[RW]

Public Class Methods

cached(*args) click to toggle source

Takes the same arguments as MarcExtractor.new, but will re-use an existing cached MarcExtractor already created with given initialization arguments, if available.

This can be used to increase performance of indexing routines, as MarcExtractor creation has been shown via profiling/benchmarking to be expensive.

Cache is thread-local, so should be thread-safe.

You should not modify the state of any MarcExtractor retrieved via cached, as the MarcExtractor will be re-used and shared (possibly between threads even!). We try to use ruby freeze to keep you from doing so, although if you try hard enough you can surely find a way to do something you shouldn't.

extractor = MarcExtractor.cached("245abc:700a", :separator => nil)
# File lib/traject/marc_extractor.rb, line 199
def self.cached(*args)
  cache = (Thread.current[:marc_extractor_cached] ||= Hash.new)
  return (cache[args] ||= Traject::MarcExtractor.new(*args).freeze)
end
new(spec, options = {}) click to toggle source

First arg is a specification for extraction of data from a MARC record. Specification can be given in two forms:

* a string specification like "008[35]:020a:245abc", see top of class
  for examples. A string specification is most typical argument.
* The output of a previous call to MarcExtractor.parse_string_spec(string_spec),
  a 'pre-parsed' specification.

Second arg is options:

:separator

default ' ' (space), what to use to separate subfield values when joining strings

:alternate_script

default :include, include linked 880s for tags that match spec. Also:

  • false => do not include.

  • :only => only include linked 880s, not original

# File lib/traject/marc_extractor.rb, line 134
def initialize(spec, options = {})
  self.options = {
      :separator        => SPACE,
      :alternate_script => :include
  }.merge(options)

  self.spec_set = SpecSet.new(spec)


  # Tags are "interesting" if we have a spec that might cover it
  # By default, interesting tags are those represented by keys in spec_hash.
  # Add them unless we only care about alternate scripts.
  unless options[:alternate_script] == :only
    self.spec_set.tags.each { |tag| show_interest_in_tag(tag) }
  end

  # If we *are* interested in alternate scripts, add the 880
  if options[:alternate_script] != false
    @fetch_alternate_script = true
    show_interest_in_tag(ALTERNATE_SCRIPT_TAG)
  else
    @fetch_alternate_script = false
  end

  @interesting_tags_list = @interesting_tags_hash.keys

  self.freeze
end

Public Instance Methods

collect_matching_lines(marc_record) { |field, spec, extractor| ... } click to toggle source

line each_matching_line, takes a block to process each matching line, but collects results of block into an array – flattens any subarrays for you!

Useful for re-use of this class for custom processing

yields the MARC Field, the MarcExtractor::Spec object, the MarcExtractor object.

# File lib/traject/marc_extractor.rb, line 243
def collect_matching_lines(marc_record)
  results = []
  self.each_matching_line(marc_record) do |field, spec, extractor|
    results.concat [yield(field, spec, extractor)].flatten
  end
  return results
end
collect_subfields(field, spec) click to toggle source

Pass in a marc data field and a Spec object with extraction instructions, returns an ARRAY of one or more strings, subfields extracted and processed per spec. Takes account of options such as :separator

Always returns array, sometimes empty array.

# File lib/traject/marc_extractor.rb, line 258
def collect_subfields(field, spec)
  subfields = field.subfields.collect do |subfield|
    subfield.value if spec.includes_subfield_code?(subfield.code)
  end.compact

  return subfields if subfields.empty? # empty array, just return it.

  if options[:separator] && spec.joinable?
    subfields = [subfields.join(options[:separator])]
  end

  return subfields
end
control_field?(field) click to toggle source
# File lib/traject/marc_extractor.rb, line 283
def control_field?(field)
  # should the MARC gem have a more efficient way to do this,
  # define #control_field? on both ControlField and DataField?
  return field.kind_of? MARC::ControlField
end
each_matching_line(marc_record) { |field, spec, self| ... } click to toggle source

Yields a block for every line in source record that matches spec. First arg to block is MARC::DataField or ControlField, second is the MarcExtractor::Spec that it matched on. May take account of options such as :alternate_script

Third (optional) arg to block is self, the MarcExtractor object, useful for custom implementations.

# File lib/traject/marc_extractor.rb, line 227
def each_matching_line(marc_record)
  marc_record.fields(interesting_tags).each do |field|

    specs_covering_field(field).each do |spec|
        yield(field, spec, self)
    end

  end
end
extract(marc_record) click to toggle source

Returns array of strings from a MARC::Record, extracted values. May be empty array.

# File lib/traject/marc_extractor.rb, line 206
def extract(marc_record)
  results = []

  self.each_matching_line(marc_record) do |field, spec|
    if control_field?(field)
      results << (spec.bytes ? field.value.byteslice(spec.bytes) : field.value)
    else
      results.concat collect_subfields(field, spec)
    end
  end

  return results
end
freeze() click to toggle source
Calls superclass method
# File lib/traject/marc_extractor.rb, line 289
def freeze
  self.options.freeze
  self.spec_set.freeze
  super
end
interesting_tag?(tag) click to toggle source

Check to see if a tag is interesting (meaning it may be covered by a spec and the passed-in options about alternate scripts)

# File lib/traject/marc_extractor.rb, line 172
def interesting_tag?(tag)
  return @interesting_tags_hash.include?(tag)
end
interesting_tags() click to toggle source

All the “interesting” tags

# File lib/traject/marc_extractor.rb, line 177
def interesting_tags
  @interesting_tags_list
end
show_interest_in_tag(tag) click to toggle source

Declare that we're interested in a tag

# File lib/traject/marc_extractor.rb, line 165
def show_interest_in_tag(tag)
  @interesting_tags_hash      ||= {}
  @interesting_tags_hash[tag.freeze] = true
end
specs_covering_field(field) click to toggle source

Find Spec objects, if any, covering extraction from this field. Returns an array of 0 or more MarcExtractor::Spec objects

Returns an empty array in case of no matching extraction specs.

# File lib/traject/marc_extractor.rb, line 277
def specs_covering_field(field)
  return [] unless interesting_tag?(field.tag)
  self.spec_set.specs_matching_field(field, @fetch_alternate_script)
end