Field cardinality is not taken into account on field mapping

The patch logic sounds good. I'll test and include it in next beta (hopefully this week... things are always delayed... :( ).

Merged. Thank you for the report and the fix. :)

Applied by commit #c016c35143ed4b02bd2ca5bdc46f8fee843ff672 and credited.

In fact, I read a bit too fast. The fix does not change anything and the tests are passing.

I took time to investigate. First, the patch you propose does nothing. It sets a value that is not used anywhere. That's why automated tests were still passing and I though the fix was working.

After reviewing the fix again, I saw it was useless so I manually tested this dataset to make sure everything works fine (json dataset):

{
"a":{
  "id": "a",
  "title": "element 1",
  "field1": "toto",
  "field2": "titi",
  "field3": "tutu"
},
"b":{
  "id": "b",
  "title": "element 2",
  "field1": "",
  "field2": "titi",
  "field3": "tutu"
},
"c":{
  "id": "c",
  "title": "element 3",
  "field1": 0,
  "field2": "titi",
  "field3": "tutu"
},
"d":{
  "id": "d",
  "title": "element 4",
  "field1": null,
  "field2": "titi",
  "field3": "tutu"
},
"e":{
  "id": "e",
  "title": "element 5",
  "field2": "titi",
  "field3": "tutu"
},
"f":{
  "id": "f",
  "title": "element 6",
  "field1": null,
  "field3": "tutu"
},
"g":{
  "id": "g",
  "title": "element 7",
  "field1": [],
  "field2": null,
  "field3": "tutu"
},
"h":{
  "id": "h",
  "title": "element 8",
  "field1": ["a", "b", "c"],
  "field2": null,
  "field3": "tutu"
},
"i":{
  "id": "i",
  "title": "element 9",
  "field1": [""],
  "field2": ["", "", "x"],
  "field3": "tutu"
},
"j":{
  "id": "j",
  "title": "element 10",
  "field1": [" "],
  "field2": ["", null, "x"],
  "field3": "tutu"
}
}

I used a test external entity type with a field "multitext" that was mapped using multiple mapping: first field was set to "field1", second to "field2" and third to "field3". All those 3 mappings were using the "simple" mapping type with no data processors. The loading order was set to "1,2,3" with "stop at first non-empty mapping".

Results are the expected ones for "multitext":
element1: "toto"
element2: "titi"
element3: 0
element4: "titi"
element5: "titi"
element6: "tutu"
element7: "tutu"
element8: ["a", "b", "c"]
element9: "x"
element10: " "

For me, all is working as designed. I cannot reproduce your issue. Could you elaborate more on your settings and provide your exact mapping for a given dataset, just like I did?

OK, everything work as designed, it is just a different problem of mapping than the one you describe. I agree things are not easy to understand, and the documentation and setting descriptions could be better.

With your example on the "b" entity, the multiple mapping first tries to get "title.title" which does not exist. Then it tries to get "title.*.title" which is a non-null array of values. It will be assigned to the title field. So it works as expected. To convince you, here is another example:

{
  "id": "c",
  "title": {
    "title": "element 3 - title 1",
    "0": {
      "title":  "element 3 - title 2"
    },
    "1": {
      "title":  "element 3 - title 3"
    }
  }
}

Try it and you will see only "element 3 - title 1" will be used as title.

The problem you meet is not related to the multiple mapping. It is how external entity provides data to Drupal fields: it currently does not take into account the field cardinality (while it should). The "title" field is supposed to only have one value but with your dataset, it is filled with a set of values. I should take into account the field cardinality configuration to truncate array of values.

So I'll update the issue description. Meanwhile, if it works for you, you may use a different second mapping: "title.0.title", which should only keep the first title value. Hopefully, you don't have in your dataset a first "null" title when there is a list of titles...
There may be other ways to solve you current problem, maybe using conditional mapping but I don't have time to investigate more that, and anyway, the real problem behind is different and solving it will solve your problem as well.

I committed a fix to support field cardinality. Please confirm the fix works as expected and mark the issue reviewed if so.

You're welcome! Thanks for confirming the fix. ;)