# DocJoinFilter **`class DocJoinFilter(config)`** Bases: [`Filter`](squirro.lib.nlp.steps.filters.Filter.md#squirro.lib.nlp.steps.filters.Filter) The doc_join [`Filter`](squirro.lib.nlp.steps.filters.Filter.md#squirro.lib.nlp.steps.filters.Filter) joins multiple [`Document`](../technical/libnlp/base.md#squirro.lib.nlp.document.Document) into one [`Document`](../technical/libnlp/base.md#squirro.lib.nlp.document.Document) via id or a specified field. **Note** - This step assumes the fields arrive in a pre-sorted state, so that when the value of the join_field (if specified) changes, a new [`Document`](../technical/libnlp/base.md#squirro.lib.nlp.document.Document) is created. Fields from subsequent Documents with the same join_field will be appended until the join_field changes again. **Input** - No specific input fields type required. **Output** - The output field type is going to be of type [`list`](https://docs.python.org/3.11/library/stdtypes.html#list) [ input field type ]. Parameters - `type` ([`str`](https://docs.python.org/3.11/library/stdtypes.html#str)) – doc_join - `fields` ([`list`](https://docs.python.org/3.11/library/stdtypes.html#list)`, ``[``]`) – Fields to join - `join_field` ([`str`](https://docs.python.org/3.11/library/stdtypes.html#str)`, ``'id'`) – Field to join on **Example** ```json { "step": "filter", "type": "doc_join", "fields":["sentences"] } ``` Methods SummaryMethods Documentation **`process(docs)`** Process a set of documents Parameters `docs` (`generator``(`[`Document`](../technical/libnlp/base.md#squirro.lib.nlp.document.Document)`)`) – Generator of documents Returns Generator of processed documents Return type generator([Document](../technical/libnlp/base.md#squirro.lib.nlp.document.Document)) **`train(docs)`** Train on a step of a set of documents Parameters `docs` (`generator``(`[`Document`](../technical/libnlp/base.md#squirro.lib.nlp.document.Document)`)`) – Generator of documents Returns Generator of processed documents Return type generator([Document](../technical/libnlp/base.md#squirro.lib.nlp.document.Document))