Metamorph collectors

In the case that an output depends on the values from more then one literal, we need to collect literals. Collectors are defined under the tag, just as tags. tags are be put inside the respective collectors to indicate which literals are to be collected. The following paragraphs briefly introduce the different collectors available. Note that all types of collectors except can be nested. To all types of collectors post-processing steps can be added by using the <postprocess> tag.

For more information, see:

• Metamorph Cookbook
• test-cases (https://github.com/culturegraph/metafacture-core/tree/master/src/test/java/org/culturegraph/mf/morph/collectors)
• Metamorph schema (https://github.com/culturegraph/metafacture-core/tree/master/src/main/resources/schema/metamorph.xsd).

#combine

<combine> is used to build one output literal from a combination of input literals. The following example for instance collects the sur- and forename which are stored in separate literals to combine them according to the pattern 'surname, forename'.

<combine name="gnd:variantNameForThePerson" value="${surname}, ${forename}">
  <data source="028A.a" name="surname" />
  <data source="028A.d" name="forename"/>
</combine>

There are several important points to note: By default waits until at least one value from each <data> tag is received. If the collection is not complete on record end, no output is generated. After each output, the state of <combine> is reset. If one <data> tag receives literals repeatedly before the collection is complete only the last value will be retained.

The standard behavior of <combine> can be controlled with several arguments:

flushWith

flushWith= "name" generates output on the occurrence of any literal or entity with name. Variables in the output pattern which are not yet bound to a value, are replaced with the empty string. Use flushWith="record" to set the record end as output trigger. ##reset reset="false" disables the reset after output. ##sameEntity sameEntity="true" will reset the <combine> after each entity end and thus enforce combinations stemming from the same entities only. Note that the implementation only executes a reset if actually needed. Using sameEntity="true" has thus no negative impact on performance.

concat

collects all received values and concatenates them on record end. flushWith="name" can be used to write the concatenation at the occurrence of any literal or entity name.

in: ("data1", "a"), ("data2", "b"), ("data2", "c"),("data1", "d"),
<concat delimiter=", " name="concat" prefix="{" postfix="}">
   <data source="data1" />
   <data source="data2" />
</concat>
out: ("concat", "{a,b,c,d}")

Note: The values are concatenated in the order they appear in the input and not in the order of the data sources.

choose

collects all received values and emits the most preferred one on record end. Preference is assigned according to the order the data sources appear within the choose tag. Eligible arguments are sameEntity and flushWith.

in: ("data1", "a"), ("data2", "b")
<choose name="data">
   <data source="data1" />
   <data source="data2" />
</choose>
out: ("data", "a")

if, however ("data1", "a") was missing, the output would be ("data", "b").

group

Use it to set name, value or both only once for an entire group of data or collect (combine, choose, etc. ) tags.

tuples

collects literals from the enclosed sources and emits all possible combinations of them when flushed. The minN option is used to set a minimum of different sources to be received. If less than minN are received no tuples are emitted.

square

All unordered 2-tuples are constructed.

in: ("data", "a"), ("data", "b"), ("data", "c")
<square delimiter="," name="square" prefix="{" postfix="}">
   <data source="data" />
</square>
out: ("square", "{a,b}"), ("square", "{a,c}"),("square", "{b,c}")

Entity

collects literals to rearrange them as an entity. Use the argument name to assign a name to the entity. Further arguments are sameEntity, flushWith and reset. Note that the <entity> tag can only appear as child of the <rules> tag or another entity tag as it does not output a literal.