Class: Traject::MarcExtractor

Inherits:
Object
  • Object
show all
Defined in:
lib/traject/marc_extractor.rb,
lib/traject/marc_extractor_spec.rb

Overview

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:

  • {tag}{|indicators|}{subfields}
  • tag is three chars (usually but not neccesarily numeric)
  • indicators are optional two chars enclosed in pipe ('|') characters,
  • subfields are optional list of chars (alphanumeric)

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
  • subfields and indicators can only be provided for marc data/variable fields
  • byte slice can only be provided for marc control fields (generally tags less than 010)

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.

Defined Under Namespace

Classes: Spec, SpecSet

Constant Summary collapse

ALTERNATE_SCRIPT_TAG = 
'880'.freeze
SUBFIELD_6 = 
'6'.freeze
EMPTY = 
[].freeze
SPACE = 
' '.freeze

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(spec, options = {}) ⇒ MarcExtractor

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



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# 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

Instance Attribute Details

#options ⇒ Object

Returns the value of attribute options.



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# File 'lib/traject/marc_extractor.rb', line 110

def options
  @options
end

#spec_set ⇒ Object

Returns the value of attribute spec_set.



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# File 'lib/traject/marc_extractor.rb', line 110

def spec_set
  @spec_set
end

Class Method Details

.cached(*args) ⇒ Object

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)



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# 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

Instance Method Details

#collect_matching_lines(marc_record) ⇒ Object

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.



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# 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) ⇒ Object

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.



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# 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) ⇒ Boolean

Returns:

  • (Boolean) 


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# 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) ⇒ Object

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.



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# 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) ⇒ Object

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



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# 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 ⇒ Object 



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# File 'lib/traject/marc_extractor.rb', line 289

def freeze
  self.options.freeze
  self.spec_set.freeze
  super
end

#interesting_tag?(tag) ⇒ Boolean

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

Returns:

  • (Boolean) 


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# File 'lib/traject/marc_extractor.rb', line 172

def interesting_tag?(tag)
  return @interesting_tags_hash.include?(tag)
end

#interesting_tags ⇒ Object

All the "interesting" tags



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# File 'lib/traject/marc_extractor.rb', line 177

def interesting_tags
  @interesting_tags_list
end

#show_interest_in_tag(tag) ⇒ Object

Declare that we're interested in a tag



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# 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) ⇒ Object

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.



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# 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