NAME

       UR::DataSource::Filesystem - Get and save objects to delimited text files

SYNOPSIS

         # Create an object for the data file
         my $people_data = UR::DataSource::Filesystem->create(
             columns => ['person_id','name','age','street_address'],
             sorted_columns => ['age','person_id'],
             path => '/var/lib/people/$state/$city/people.txt',
             delimiter        => "\t", # between columns in the file
             record_separator => "\n", # between lines in the file
         );

         # Define an entity class for the people in the file
         class MyProgram::Person {
             id_by => 'person_id',
             has => [
                 name           => { is => 'String' },
                 age            => { is => 'Number' },
                 street_address => { is => 'String' },
                 city           => { is => 'String' },
                 state          => { is => 'String' },
             ],
             data_source_id => $people_data->id,
         };

         # Get all people that live in any city named Springfield older than 40
         my @springfielders = MyProgram::Person->get(city => 'Springfield', 'age >' => 40);

DESCRIPTION

       A Filesystem data source object represents one or more files on the fileystem.  In the
       simplest case, the object's 'path' property names a file that stores the data.

   Properties
       These properties determine the configuration for the data source.

       path <string>
           path is a string representing the path to the files.  Besides just being a simple
           pathname to one file, the string can also be a specification of many similar files, or
           a directory containing multiple files.  See below for more information about 'path'

       record_separator <string>
           The separator between lines in the file.  This gets stored in $/ before calling
           getline() to read data.  The default record_separator is "\n".

       delimiter <string>
           The separator between columns in the file.  It is used to construct a regex with qr()
           to split() a line into a list of values.  The default delimiter is '\s*,\s*', meaning
           that the file is separated by commas.  Another common value would be "\t" for tabs.

       columns <ARRAY>
           A listref of column names in the file.  Just as SQL tables have columns, Filesystem
           files also have named columns so the system knows how to read the file data into
           object properties.  A Filesystem data source does not need to specify named columns if
           the 'columns_from_header' property is true.

           Classes that use the Filesystem data source attach their properties to the data
           source's columns via the 'column_name' metadata.  Besides the columns directly named
           in the 'columns' list, two additional column-like tokens may be used as a column_name:
           '__FILE__' and '$.'.  __FILE__ means the object's property will hold the name of the
           file the data was read from.  $. means the value will be the input line number from
           the file.  These are useful when iterating over the contents of a file.  Since these
           two fake columns are always considered "sorted", it makes reading from the file faster
           in some cases.  See the 'sorted_columns' discussion below for more information.

       sorted_columns <ARRAY>
           A listref of column names that the file is sorted by, in the order of the sorting.  If
           a column is sorted in descending order, put a minus (-) in front of the name.  If the
           file is sorted by multiple columns, say first by last_name and then by first_name,
           then include them both:

             sorted_columns => ['last_name','first_name']

           The system uses this information to know when to stop reading if a query is done on a
           sorted column.  It's also used to determine whether a query done on the data source
           matches the sort order of the file.  If not, then the data must be gathered in two
           passes.  The first pass finds records in the file that match the filter.  After that,
           the matching records are sorted in the same way the query is requesting before
           returning the data to the Context.

           The Context expects incoming data to always be sorted by at least the class' ID
           properties.  If the file is unsorted and the caller wants to be able to iterate over
           the data, then it is common to have the class' ID properties specified like this:

             id_by => [
                 file => { is => 'String', column_name => '__FILE__' },
                 line => { is => 'Integer', column_name => '$.' },
             ]

           Otherwise, it will need to read in the whole file and sort the contents before
           returning the first row of data from its iterator.

       columns_from_header <boolean>
           If true, the system will read the first line of the file to determine what the column
           names are.

       header_lines <integer>
           The number of lines at the top of the file that do not contain entity data.  When the
           file is opened, this number of lines are skipped before reading data.  If the
           columns_from_header flag is true, the header_lines value should be at least 1.

       handle_class <string>
           Which class to use for reading and writing to the file.  The default is IO::File.  Any
           other value must refer to a class that has the same interface as IO::File, in
           particular: new, input_line_number, getline, tell, seek and print.

   Path specification
       Besides refering to just one file on the filesystem, the path spec is a recipe for finding
       files in a directory tree.  If a class using a Filesystem data source does not have
       'table_name' metadata, then the path specification must resolve to file names.
       Alternatively, classes may specify their 'table_name' which is interpreted as a file
       within the directory indicated by the path specification.

       Three kinds of special tokens can also appear in a file spec:

       $property
           When querying, the system will extract the value (or values, for an in-clause) of
           $property from the BoolExpr when constructing the pathname.  If the BoolExpr does not
           have a value for that property, then the system will do a shell glob to find the
           possible values.  For example, given this path spec and query:

             path => '/var/people/$state/$city/people.txt'
             my @people = MyProgram::People->get(city => 'Springfield', 'age >' => 40);

           it would find the data files using the glob expression

             /var/people/*/Springfield/people.txt

           It also knows that any objects coming from the file

             /var/people/CA/Springfield/people.txt

           must have the value 'CA' for their 'state' property, even though that information is
           not in the contents of the file.

           When committing changes back to the file, the object property values are used to
           determine which file it should be saved to.

           The property name can also be wrapped in braces:

             /var/people/${state}_US/city_${city}/people.txt

       &method
           The replacement value is resolved by calling the named method on the subject class of
           the query.  The method is called like this:

             $replacement = $subject_class->$method( $boolexpr_or_object);

           During a query, the method is passed a BoolExpr; during a commit, the method is passed
           an object.  It must return a string.

           The method name can also be wrapped in braces:

             /&{resolve_prefix}.dir/people.txt

       *, ?
           Literal shell glob wildcards are honored when finding files, but their values are not
           used to supply values to objects.

   Environment Variables
       If the environment variable $UR_DBI_MONITOR_SQL is true, then the Filesystem data source
       will print information about the queries it runs.

INHERITANCE

         UR::DataSource

SEE ALSO

       UR, UR::DataSource