Class: SqlToCsvStream::JsonEnumerator

Inherits:
Object
  • Object
show all
Defined in:
lib/sql_to_csv_stream/json_enumerator.rb

Constant Summary collapse

JSON_COPY_OPTIONS = 
{ format: 'TEXT' }.freeze

Instance Method Summary collapse

Constructor Details

#initialize(object, connection: PostgresqlCopyEnumerator.default_connection) ⇒ JsonEnumerator

Returns a new instance of JsonEnumerator.



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# File 'lib/sql_to_csv_stream/json_enumerator.rb', line 7

def initialize(object, connection: PostgresqlCopyEnumerator.default_connection)
  sql = (object.respond_to?(:to_sql) ? object.to_sql : object.to_s).chomp(';')
  # The inspiration of this magic was:
  # https://dba.stackexchange.com/questions/90482/export-postgres-table-as-json?newreg=0b667caa47c34084bee6c90feec5e4be
  #
  # The idea is to stream with postgresql COPY command with the TEXT format.
  # To do this, we need to convert each row in the postgresql query result into JSON first.
  # This query does exactly that by using ROW_TO_JSON.
  # There is one edge case when text in that JSON contains backslashes -- to not break our
  # escaping, we need to manually add additional escape-backslashes.
  sql = "SELECT REGEXP_REPLACE(ROW_TO_JSON(t)::TEXT, '\\\\', '\\', 'g') FROM (#{sql}) AS t"
  @copy_enum = PostgresqlCopyEnumerator.new(sql, connection: connection, copy_options: JSON_COPY_OPTIONS)
end

Instance Method Details

#each(&stream) ⇒ Object 



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# File 'lib/sql_to_csv_stream/json_enumerator.rb', line 21

def each(&stream)
  stream.yield('[')

  # do not prepend a comma on the very first item
  # to not break json syntax
  prepend_comma = false
  @copy_enum.each do |line|
    line = ',' + line if prepend_comma
    prepend_comma = true
    stream.yield(line.chomp)
  end

  stream.yield("]\n")
end